fix: add mypy checking + 'py.typed' file (#290)

diff --git a/mypy.ini b/mypy.ini
new file mode 100644
index 0000000..5663b40
--- /dev/null
+++ b/mypy.ini
@@ -0,0 +1,7 @@
+[mypy]
+python_version = 3.6
+namespace_packages = True
+ignore_missing_imports = True
+
+[mypy-google.protobuf]
+ignore_missing_imports = True