This was already the configuration in Travis and it's been working fine. It's
confusing that Travis was using a different configuration than the default
when you run tests/mypy_test.py yourself.
* Add flags to pass on --warn-unused-ignores and --no-implicit-optional to mypy
* Make implicit Optional explicit in arg types (2and3 part)
* Convert {stdlib,third_party}/2 to explicit Optional
This simplifies running flake8 tests and reduces the amount of F821 errors
reported (flake8-pyi enables support for forward references in *.pyi files).
The error code is left disabled until I clean up the remaining issues.
