Add a "mypy" tox environment which runs mypy
type checking against Cinder code.
Taken from Stephen Finucane's Nova work at
https://review.opendev.org/#/c/676208/
Added "show_error_codes" and "pretty" options.
Generates an html report in ./mypy-report/
This adds stubs for oslo.i18n, so that _() calls
are annotated as intended. It may be possible to
do this with less .pyi files carried along here.
Change-Id: I2589d22c1f16f2e177d34730a520591743c0c1e3
6e53c264b6