Fixes for Python 3.9+ compatibility

[walton98]

#12 assumes that end_lineno will not be set for single-line strings. However, as of Python 3.9 (python/cpython@b7e9525f9c7ef02a1d2ad8253afdeb733b0951d4) these fields are instead defaulted to None.

Also add testing for Python 3.9/3.10 and drop unmaintained flake8-strict package.