Let close_fds be True on all platforms

Since Python 3.7, subprocess.Popen supports close_fds=True on all
platforms, including Windows, and it is the default, including
when arguments for standard streams have non-None values passed.
3.7 is the lowest version of Python that GitPython supports. So
this omits the close_fds=True argument from the calls where it was
present. This has the same effect (in 3.7 and higher) as passing
close_fds=True.

When the the close_fd argument was added to the Popen call in
git.cmd.Git.execute in 1ee2afb, Python 2 was still supported. In
Python 2, close_fds defaulted to False. This appears to be the
reason it had been passed explicitly. It was conditioned on being
on a Unix-like system because having it True on Windows would
prevent stdin, stdout, or stderr redirection.

git/cmd.py

@@ -1001,7 +1001,6 @@ def execute(
                     stderr=PIPE,
                     stdout=stdout_sink,
                     shell=shell,
-                    close_fds=(os.name == "posix"),  # Unsupported on Windows.
                     universal_newlines=universal_newlines,
                     creationflags=PROC_CREATIONFLAGS,
                     **subprocess_kwargs,

git/index/fun.py

@@ -102,7 +102,6 @@ def run_commit_hook(name: str, index: "IndexFile", *args: str) -> None:
             stdout=subprocess.PIPE,
             stderr=subprocess.PIPE,
             cwd=index.repo.working_dir,
-            close_fds=(os.name == "posix"),
             creationflags=PROC_CREATIONFLAGS,
         )
     except Exception as ex: