config: Fix mypy error for git.exc.

Error was:

  conservancy_beancount/config.py:142: error: Trying to read deleted variable "exc"

Not entirely sure what's causing this, but since GitPython also imports the
exceptions at the root of the package with "from git.exc import *", we can
import them from there instead, which seems to appease mypy.

conservancy_beancount/config.py

@@ -139,7 +139,7 @@ class Config:
         """
         try:
             return git.Repo(self.file_config['Beancount']['books dir'])
-        except (KeyError, git.exc.GitError):
+        except (KeyError, git.GitError):
             return None
 
     def cache_dir_path(self, name: str='conservancy_beancount') -> Optional[Path]: