Add mypy tox env

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

tox.ini

@@ -169,6 +169,13 @@ deps = bindep
 commands = bindep {posargs}
 usedevelop = False
 
+[testenv:mypy]
+description =
+  Run type checks.
+envdir = {toxworkdir}/pep8
+commands =
+  bash tools/mypywrap.sh {posargs}
+
 [flake8]
 # Following checks are ignored on purpose.
 #