Extract all "import gc" to module level

The gc module was already imported at module scope in
git/repo/base.py, since f1a82e4 (#555). Importing the top-level
git module or any submodule of it runs that import statement.
Because the gc module is already imported, reimporting it is fast.

Imports that there is no specific reason to do locally should be at
module scope. Having them local decreased readability, in part
because of how black inserts a black line between them and
gc.collect() calls they are imported to allow.

An alternative to this change would be to remove the preexisting
top-level "import gc" (there is also another one in the test suite)
and replace it with a local import as well. I am unsure if that
would affect performance and, if so, whether the effect would be
good or bad, since the small delay of the import might potentially
be less desirable to an applicaion if it occurs while the work of
the application is already in progress.

If a gc.collect() call runs as a consequence of a finally block or
__del__ method being called during interpreter shutdown, then in
(very) rare cases the variable may have been set to None. But this
does not appear to have been the intent behind making the imports
local. More importantly, a local import should not be expected to
succeed, or the imported module usable, in such a situation.

git/objects/submodule/base.py

@@ -1,6 +1,7 @@
 # This module is part of GitPython and is released under the
 # 3-Clause BSD License: https://opensource.org/license/bsd-3-clause/
 
+import gc
 from io import BytesIO
 import logging
 import os
@@ -1079,8 +1080,6 @@ def remove(
                     self._clear_cache()
                     wtd = mod.working_tree_dir
                     del mod  # Release file-handles (Windows).
-                    import gc
-
                     gc.collect()
                     rmtree(str(wtd))
                 # END delete tree if possible

test/performance/test_commit.py

@@ -5,6 +5,7 @@
 
 """Performance tests for commits (iteration, traversal, and serialization)."""
 
+import gc
 from io import BytesIO
 from time import time
 import sys
@@ -17,8 +18,6 @@
 
 class TestPerformance(TestBigRepoRW, TestCommitSerialization):
     def tearDown(self):
-        import gc
-
         gc.collect()
 
     # ref with about 100 commits in its history.

test/performance/test_streams.py

@@ -3,6 +3,7 @@
 
 """Performance tests for data streaming."""
 
+import gc
 import os
 import subprocess
 import sys
@@ -92,8 +93,6 @@ def test_large_data_streaming(self, rwrepo):
 
             # del db file so git has something to do.
             ostream = None
-            import gc
-
             gc.collect()
             os.remove(db_file)
 

test/test_base.py

@@ -3,6 +3,7 @@
 # This module is part of GitPython and is released under the
 # 3-Clause BSD License: https://opensource.org/license/bsd-3-clause/
 
+import gc
 import os
 import sys
 import tempfile
@@ -20,8 +21,6 @@
 
 class TestBase(_TestBase):
     def tearDown(self):
-        import gc
-
         gc.collect()
 
     type_tuples = (

test/test_docs.py

@@ -3,6 +3,7 @@
 # This module is part of GitPython and is released under the
 # 3-Clause BSD License: https://opensource.org/license/bsd-3-clause/
 
+import gc
 import os
 import sys
 
@@ -16,8 +17,6 @@
 
 class Tutorials(TestBase):
     def tearDown(self):
-        import gc
-
         gc.collect()
 
     # ACTUALLY skipped by git.util.rmtree (in local onerror function), from the last call to it via

test/test_git.py

@@ -3,6 +3,7 @@
 # This module is part of GitPython and is released under the
 # 3-Clause BSD License: https://opensource.org/license/bsd-3-clause/
 
+import gc
 import inspect
 import logging
 import os
@@ -34,8 +35,6 @@ def setUpClass(cls):
         cls.git = Git(cls.rorepo.working_dir)
 
     def tearDown(self):
-        import gc
-
         gc.collect()
 
     def _assert_logged_for_popen(self, log_watcher, name, value):

test/test_quick_doc.py

@@ -1,14 +1,14 @@
 # This module is part of GitPython and is released under the
 # 3-Clause BSD License: https://opensource.org/license/bsd-3-clause/
 
+import gc
+
 from test.lib import TestBase
 from test.lib.helper import with_rw_directory
 
 
 class QuickDoc(TestBase):
     def tearDown(self):
-        import gc
-
         gc.collect()
 
     @with_rw_directory

test/test_remote.py

@@ -3,6 +3,7 @@
 # This module is part of GitPython and is released under the
 # 3-Clause BSD License: https://opensource.org/license/bsd-3-clause/
 
+import gc
 import os
 import os.path as osp
 from pathlib import Path
@@ -105,8 +106,6 @@ def assert_received_message(self):
 
 class TestRemote(TestBase):
     def tearDown(self):
-        import gc
-
         gc.collect()
 
     def _print_fetchhead(self, repo):

test/test_repo.py

@@ -3,6 +3,7 @@
 # This module is part of GitPython and is released under the
 # 3-Clause BSD License: https://opensource.org/license/bsd-3-clause/
 
+import gc
 import glob
 import io
 from io import BytesIO
@@ -72,8 +73,6 @@ def tearDown(self):
             if osp.isfile(lfp):
                 raise AssertionError("Previous TC left hanging git-lock file: {}".format(lfp))
 
-        import gc
-
         gc.collect()
 
     def test_new_should_raise_on_invalid_repo_location(self):

test/test_submodule.py

@@ -2,6 +2,7 @@
 # 3-Clause BSD License: https://opensource.org/license/bsd-3-clause/
 
 import contextlib
+import gc
 import os
 import os.path as osp
 from pathlib import Path
@@ -61,8 +62,6 @@ def update(self, op, cur_count, max_count, message=""):
 
 class TestSubmodule(TestBase):
     def tearDown(self):
-        import gc
-
         gc.collect()
 
     k_subm_current = "c15a6e1923a14bc760851913858a3942a4193cdb"