Fix to allow installation with Python 3.8

[steffengraber]

This is a fix to allow installation with python 3.8.
After building NEST with python 3.8, an import nest returns
the following error.

Traceback (most recent call last):
  File "nest-simulator_1574171714961/test_tmp/run_test.py", line 2, in <module>
    import nest
  File "nest-simulator_1574171714961/lib/python3.8/site-packages/nest/__init__.py", line 30, in <module>
    from .hl_api import *      # noqa
  File "nest-simulator_1574171714961/lib/python3.8/site-packages/nest/hl_api.py", line 40, in <module>
    _il.import_libs(__file__, globals(), 'lib', ignore=_ignore_modules)
  File "nest-simulator_1574171714961/lib/python3.8/site-packages/nest/import_libs.py", line 73, in import_libs
    code = compile(module, mod_file, 'exec')
TypeError: required field "type_ignores" missing from Module

There is a further explanation here:
https://bugs.python.org/issue35894

[steffengraber]

This PR fixes #1337.

[heplesser]

@steffengraber Thank you for the fix! I have a few small comments in the code.