python: add mypy config

Formalize the options used for checking the python library. You can run
mypy from the directory that mypy.ini is in by typing `mypy qemu/`.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-id: 20201009175123.249009-2-jsnow@redhat.com
[Edit: Added newline; thanks Bin Meng --js]
Signed-off-by: John Snow <jsnow@redhat.com>

python/mypy.ini

+[mypy]
+strict = True
+python_version = 3.6
+warn_unused_configs = True