add __all__ to explicitly declare exports of beangulp

Without this, mypy will complain when importing Importer from beangulp.

beangulp/__init__.py

@@ -18,14 +18,28 @@
 from beancount import loader
 
 from beangulp import archive
-from beangulp import cache  # noqa: F401
+from beangulp import cache
 from beangulp import exceptions
 from beangulp import extract
 from beangulp import identify
 from beangulp import utils
 from beangulp.importer import Importer, ImporterProtocol, Adapter
 
 
+__all__ = [
+    "Adapter",
+    "Importer",
+    "ImporterProtocol",
+    "Ingest",
+    "archive",
+    "cache",
+    "exceptions",
+    "extract",
+    "identify",
+    "utils",
+]
+
+
 def _walk(file_or_dirs, log):
     """Convenience wrapper around beangulp.utils.walk()