refactor: Move fully to Python 3

[taion]

No description provided.

[taion]

We should pull more of our S3 stuff into here as well.

[taion]

@c0state if this looks good to you, i'll cut a release after we merge

[sloria]

Running pyupgrade gave this

diff --git a/flask_annex/file.py b/flask_annex/file.py
index 51c0964..5812bee 100644
--- a/flask_annex/file.py
+++ b/flask_annex/file.py
@@ -88,7 +88,7 @@ class FileAnnex(AnnexBase):
 
         # Verify that we aren't trying to create the root path.
         if not os.path.exists(self._root_path):
-            raise IOError(
+            raise OSError(
                 f"root path {self._root_path} does not exist",
             )
 

Otherwise this lgtm.

[taion]

ah, it should be a FileNotFoundError anyway