``mypy`` tests can be run via runtests.py
-----------------------------------------
Currently running mypy with the NumPy stubs configured requires
either:

* Installing NumPy
* Adding the source directory to MYPYPATH and linking to the mypy.ini

Both options are somewhat inconvenient, so add a ``--mypy`` option to runtests
that handles setting things up for you. This will also be useful in the future
for any typing codegen since it will ensure the project is built before type
checking.

Add gh-17123 to NumPy 1.20.0 release notes.
DOC: Add gh-17123 to NumPy 1.20.0 release notes.
