Fix py38 build

[bertsky]

fixes #836 i.e.

• ability to build untagged commits manually using pip wheel (ensuring the version conforms to PEP 440, and to our actual documentation, by using + instead of .)
• ability to compile against current OpenCV on Python 3.8 (using numpy 1.17.5 instead of 1.17.3 which does not compile anymore)

[bertsky]

Note: CI failures look exactly like the CI failures on the 4.x head, so I guess this is unrelated.

Perhaps this is a regression in skbuild? See this recent change

[bertsky]

Perhaps this is a regression in skbuild? See this recent change

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[bertsky]

No interest at all?