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

setup.py

@@ -17,8 +17,13 @@ setup(
         exclude=["tests", "*.tests", "*.tests.*", "output", "output.*"]
     ),
     package_data={"betterproto": ["py.typed", "templates/template.py"]},
-    python_requires=">=3.7",
-    install_requires=["grpclib", "stringcase"],
+    python_requires=">=3.6",
+    install_requires=[
+        'dataclasses; python_version<"3.7"',
+        'backports-datetime-fromisoformat; python_version<"3.7"',
+        "grpclib",
+        "stringcase",
+    ],
     extras_require={"compiler": ["black", "jinja2", "protobuf"]},
     zip_safe=False,
 )