Cleaning state pollution of global variable arguments by assigning with None

[sturmianseq]

This PR aims to improve test reliability of test test_docopt_ng_more_magic_global_arguments_and_dot_access by cleaning state pollution of global variable arguments by assigning with None.

The test can fail in this way by running pip3 install pytest-repeat; python3 -m pytest --count=2 tests/test_docopt_ng.py::test_docopt_ng_more_magic_global_arguments_and_dot_access:

        global arguments
        docopt.docopt(doc, "-v file.py", more_magic=True)
>       assert arguments == {"-v": True, "-q": False, "-r": False, "--help": False, "FILE": "file.py", "INPUT": None, "OUTPUT": None}
E       AssertionError: assert {'--help': Fa...OUTPUT': None} == {'--help': Fa...v': True, ...}
E         Omitting 6 identical items, use -vv to show
E         Left contains 1 more item:
E         {'<FILE>': None}
E         Right contains 1 more item:
E         {'FILE': 'file.py'}

It may be better to clean state pollutions so that some other tests won't fail in the future due to the shared state pollution.

[codecov]

Codecov Report

Merging #20 (b1adf62) into master (de7c861) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #20   +/-   ##
=======================================
  Coverage   99.79%   99.79%           
=======================================
  Files           1        1           
  Lines         491      491           
=======================================
  Hits          490      490           
  Misses          1        1           

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[NickCrews]

Thanks @sturmianseq!! I rebased on master to ensure that CI still passed.