tests: drop pip test on the client

Closed Jörg Sonnenberger requested to merge topic/default/pip-test into branch/default

The PIP testing is fragile, various hacks are included already. It has been failing persistently in the CI and locally on OpenSuSE. Since general PIP use requires network access, it is better done during CI anyway.

Differential Revision: https://phab.mercurial-scm.org/D9928

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