dev-requirements: Bump pytest requirement for tmp_path

tmp_path was actually introduced in the 3.9.x series,
but it slipped by when the pytest version was bumped before
by pip opting to fetch a version which was newer than necessary.

dev-requirements.txt

 coverage == 4.4.0
 pep8
 pylint == 2.1.1
-pytest >= 3.8
+pytest >= 3.9.1
 pytest-cov >= 2.5.0
 pytest-datafiles
 pytest-env