Use custom branch for olm bindings with mypy

tox.ini

@@ -33,5 +33,13 @@ commands =
 
 extras = dev
 
+# The current version of python-olm that's on PyPI does not include a types marker.
+# Ideally we'd just specify a custom pip install command that uses an --index-url that
+# points to Olm's PyPI repo, but it looks like the packaging does not include py.typed in
+# wheels. https://gitlab.matrix.org/matrix-org/olm/-/merge_requests/62 is an attempt at
+# fixing this.
+deps =
+  git+https://gitlab.matrix.org/babolivier/olm.git@babolivier/py.typed_manifest#egg=python-olm&subdirectory=python
+
 commands =
   mypy matrix_content_scanner tests