Bump mautrix-python requirement

Should fix "Already sharing group session" problems

requirements.txt

@@ -4,5 +4,5 @@ alembic>=1,<2
 ruamel.yaml>=0.15.94,<0.17
 commonmark>=0.8,<0.10
 python-magic>=0.4,<0.5
-mautrix==0.6.0.alpha1
+mautrix==0.6.0.alpha2
 fbchat-asyncio>=0.6.4,<0.7