Add support for python 3.6

Changes:
- Update config and docs to reference 3.6
- Add backports of dataclasses and datetime.fromisoformat for python_version<"3.7"
- Support both 3.7 and 3.6 usages of undocumented __origin__ attribute on typing objects
- Make github ci run tests for python 3.6 as well

.gitignore

@@ -2,6 +2,8 @@
 .vscode/settings.json
 .mypy_cache
 .pytest_cache
+.python-version
+build/
 betterproto/tests/*.bin
 betterproto/tests/*_pb2.py
 betterproto/tests/*.py