merging python_version and python_full_version markers

[dimbleby]

Resolves: python-poetry#

• Added tests for changed code.
• Updated documentation for changed code.

recognise that python_version and python_full_version markers say much the same thing and often can be merged

As an example of the difference this makes, the poetry export testcase that I have added in python-poetry/poetry#5156 sees the following diff when using this fix:

- click-didyoumean==0.3.0 ; python_full_version >= "3.6.2" and python_full_version < "4.0.0" and python_version >= "3.7" and python_version < "4.0"
+ click-didyoumean==0.3.0 ; python_version >= "3.7" and python_full_version < "4.0.0"
- click==7.1.2 ; python_version >= "3.6" and python_full_version < "3.0.0" and python_version < "3.7" or python_version >= "3.6" and python_version < "3.7" and python_full_version >= "3.5.0"
+ click==7.1.2 ; python_version >= "3.6" and python_version < "3.7"
- click==8.0.3 ; python_version >= "3.7" and python_version < "4.0" or python_full_version >= "3.6.2" and python_full_version < "4.0.0" and python_version >= "3.7" and python_version < "4.0"
+ click==8.0.3 ; python_version >= "3.7" and python_version < "4.0" or python_version >= "3.7" and python_full_version < "4.0.0"

The last click marker is not quite perfect, in ideal world we'd recognise that python_version < "4.0" was the same as python_full_version < "4.0.0" and then merge things - but it's a substantial improvement.

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[dimbleby]

rebased following merge of #282 and #293, including simplification per suggestion of #294 (comment)