Support mypy 0.950

[DamianZaremba]

mypy 0.950 was released a short while ago (https://github.com/python/mypy/releases/tag/v0.950).

django-stubs currently enforces a version less than 0.950 (https://github.com/typeddjango/django-stubs/blob/master/setup.py#L23), please enable support for 0.950.

[dimbleby]

per a comment in typeddjango/djangorestframework-stubs#203

• mypy releases much more frequently than either django-stubs or djangorestframework-stubs
• although mypy might in principle release a breaking change at any point, such things seem to be very rare. (I don't know of any example where a mypy release has broken these projects, perhaps it has happened?)
• if mypy ever does release a breaking change I can easily work around that in my own project by pinning to an older version, whereas I cannot force a newer version while django*-stubs explicitly prevents me from doing so

All of which is to say: please consider removing the upper bounds on mypy in both projects.