Get typing tests to work on Python 3.9

[person142]

The typing tests currently fail on Python 3.9 and mypy 0.770. It's possible that upgrading mypy to the recently-released 0.780 will fix the issue as it has some fixes for 3.9, e.g. python/mypy#8716

Reproducing code example:

Do a

$ ./runtests.py -s tests

on Python 3.9.

Error message:

The tests will have many errors of the form:

E               ValueError: Unexpected reveal line format: /home/travis/build/numpy/numpy/builds/venv/lib/python3.9/site-packages/numpy/__init__.pyi:53: error: syntax error in type comment

(which seem potentially related to the issue linked above).

Numpy/Python version information:

• NumPy version: 1.20.0.dev0+f09b27d
• This is on Python 3.9, though I'm relaying errors seen in CI (haven't set up 3.9 locally yet)

[person142]

(Adding the 1.20.0 milestone to make sure I look into this before the release.)