Remove dependency on six to make package lighter

[hugovk]

From @pradyunsg in https://discuss.python.org:

Honestly, I think the “best” solution is to make packaging not have dependencies. This would involve dropping pyparsing and six.

Both of them are used only in packaging.requirements . pyparsing is used to generate the parser for PEP 508 requirements. six is used for getting urlib.urlparse.urlparse in a Py2-Py3 compatible way. six should be super easy to drop (PRs welcome!) ...

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What's the best way to deal with mypy for the try/except ImportError?

$ pre-commit run --all-files
Check Toml...........................................(no files to check)Skipped
Check Yaml...............................................................Passed
Fix End of Files.........................................................Passed
Trim Trailing Whitespace.................................................Passed
mypy.....................................................................Failed
- hook id: mypy
- exit code: 1

packaging/requirements.py:12: error: Name 'urlparse' already defined (by an import)
Found 1 error in 1 file (checked 12 source files)

mypy for Python 2........................................................Failed
- hook id: mypy
- exit code: 1

packaging/requirements.py:10: error: Module 'urllib' has no attribute 'parse'
packaging/requirements.py:12: error: Name 'urlparse' already defined (possibly by an import)
Found 2 errors in 1 file (checked 11 source files)

black....................................................................Passed
flake8...................................................................Passed
check-manifest...........................................................Passed

[hugovk]

Test / macOS / 3.7 (pull_request) failed due to a connection timeout error with pip install

Passed on my fork:

https://github.com/hugovk/packaging/runs/1002137291?check_suite_focus=true