www/py-gandi.cli: Update to 0.21

Remove post-extract target, removal of compiled files from sdist has
been merged upstream [1].

Note: A number of PaaS tests fail, this has been reported upstream [2].

[1] https://github.com/Gandi/gandi.cli/pull/180
[2] https://github.com/Gandi/gandi.cli/issues/215

https://github.com/Gandi/gandi.cli/blob/0.21/CHANGES.rst

Requested by: bapt