tests: look for ensurepip before using venv
Debian appears to cripple the venv module by default by
removing the associated ensurepip functionality. (The module
isn't present at all.) This caused test-install.t to fail when
using the Debian python3 unless the python3-venv package was
installed.
This commit introduces a new hghave requirement for detecting
ensurepip and makes the Python 3 install variant conditional on
its presence. This should make test-install.t pass when
using an incomplete Debian Python.
Differential Revision: https://phab.mercurial-scm.org/D7228
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