export symbols using __all__ to avoid typing errors

Mentioned in #1322

Explanation on how it works - https://github.com/microsoft/pyright/blob/main/docs/typed-libraries.md#library-interface

Example failing snippet:
```python
import pygit2

# 4:23 - error: "Repository" is not exported from module "pygit2" (reportPrivateImportUsage)
def test(repo: pygit2.Repository):
    ...
```

Author: Andrej730

pygit2/__init__.py

@@ -61,6 +61,10 @@
 from .utils import to_bytes, to_str
 
 
+__all__ = (
+    "Repository",
+)
+
 # Features
 features = enums.Feature(C.git_libgit2_features())