export symbols using __all__ to avoid typing errors

Mentioned in libgit2#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

def test(repo: pygit2.Repository):
    ...
```

Author: Andrej730

pygit2/__init__.py

@@ -61,6 +61,79 @@
 from .utils import to_bytes, to_str
 
 
+__all__ = (
+    # Low level API
+    'Object',
+    'Reference',
+    'AlreadyExistsError',
+    'Blob',
+    'Branch',
+    'Commit',
+    'Diff',
+    'DiffDelta',
+    'DiffFile',
+    'DiffHunk',
+    'DiffLine',
+    'DiffStats',
+    'GitError',
+    'InvalidSpecError',
+    'Mailmap',
+    'Note',
+    'Odb',
+    'OdbBackend',
+    'OdbBackendLoose',
+    'OdbBackendPack',
+    'Oid',
+    'Patch',
+    'RefLogEntry',
+    'Refdb',
+    'RefdbBackend',
+    'RefdbFsBackend',
+    'Repository',
+    'RevSpec',
+    'Signature',
+    'Stash',
+    'Tag',
+    'Tree',
+    'TreeBuilder',
+    'Walker',
+    'Worktree',
+    #
+    'Blame',
+    'BlameHunk',
+    'BlobIO',
+    'Payload',
+    # callbacks
+    'RemoteCallbacks',
+    'CheckoutCallbacks',
+    'StashApplyCallbacks',
+    'git_clone_options',
+    'git_fetch_options',
+    'git_proxy_options',
+    'get_credentials',
+    #
+    'Config',
+    # credentials
+    'Username',
+    'UserPass',
+    'Keypair',
+    'KeypairFromAgent',
+    'KeypairFromMemory',
+    #
+    'check_error',
+    'Passthrough',
+    'Filter',
+    'Index',
+    'IndexEntry',
+    'PackBuilder',
+    'Remote',
+    'Repository',
+    'Settings',
+    'Submodule',
+    'to_bytes',
+    'to_str',
+)
+
 # Features
 features = enums.Feature(C.git_libgit2_features())