labours: Support for Python 3.8 and 3.9

[nekketsuuu]

The package labours v10.7.2, the current latest version of labours, cannot be installed by Python 3.8 and 3.9 using pip3 install -e ./python because its dependent library seriate 1.1.2 depends on ortools 6.9.5824, which only supports Python <= 3.7.

• https://github.com/src-d/hercules/blob/v10.7.2/python/requirements.txt#L30
• https://github.com/src-d/seriate/blob/0af55a2523938eee162db56dc5ba7399785c584e/requirements.txt#L3

It seems that ortools 7.5 supports Python 3.8, and ortools 8.0 supports Python 3.9.

• https://developers.google.com/optimization/support/release_notes#announcing-the-release-of-or-tools-v7.5
• https://developers.google.com/optimization/support/release_notes#announcing-the-release-of-or-tools-v8.0

How about upgrading dependencies to support Python 3.8 and 3.9?

[MathieuDR]

Could a PR/Fork be opened for this?

[nekketsuuu]

src-d/seriate#8

[znerd]

Seems related to #365.