Read version in setup.py

[Afoucaul]

Since October 2020, pip became more strict toward version declaration. As a result, install cairocffi with the latest version (20.3.3) fails with the following error:

WARNING: Requested cairocffi>=0.9.0 from https://files.pythonhosted.org/packages/84/ca/0bffed5116d21251469df200448667e90acaa5131edea869b44a3fbc73d0/cairocffi-1.2.0.tar.gz#sha256=9a979b500c64c8179fec286f337e8fe644eca2f2cd05860ce0b62d25f22ea140 (from weasyprint), but installing version file-.cairocffi-VERSION
ERROR: Requested cairocffi>=0.9.0 from https://files.pythonhosted.org/packages/84/ca/0bffed5116d21251469df200448667e90acaa5131edea869b44a3fbc73d0/cairocffi-1.2.0.tar.gz#sha256=9a979b500c64c8179fec286f337e8fe644eca2f2cd05860ce0b62d25f22ea140 (from weasyprint) has different version in metadata: 'file-.cairocffi-VERSION'

This is mentioned in #172, which suggests using --use-deprecated=legacy-resolver. This is unstable, as this flag is intended to be remove in a future release.

On the other hand, using pip 20.2.1 works, but says:

ERROR: After October 2020 you may experience errors when installing or updating packages. This is because pip will change the way that it resolves dependency conflicts.

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As a fix, I propose to simply read the version number from cairocffi/VERSION in setup.py. I've succesfully tested this by installing my fork via pip's VCS support.

Solves #172

[liZe]

Hi! Thanks for the pull request.

We’re not sure about what the future of cairocffi will be. We (CourtBouillon, the current maintainer) are about to drop Cairo from WeasyPrint dependencies (Kozea/WeasyPrint#1232), and we’ll soon decide what is better for the future of cairocffi. Until then, we don’t really want to change the way the library works, and we prefer to keep the status quo.

[liZe]

Fixed by e17b488.