Need a newer Virtualenv for 3.6, and newer Pytest for 3.5+, so the
requirements.txt are modified to allow newer versions.

Path of tmpdir on macOS is a symlink and need to be resolved explicitly
to prevent the check from failing.

Some tests are beefed up to help pinpoint the problem more accurately.

So we can use it in the venv backend implementation.

The venv backend is used by default when "venv" and "ensurepip" are
available (i.e. Python 3.4+). This can be disabled with the
PEW_USE_VIRTUALENV environ.

Certain tests are skipped:

@skip_venv
Test makes assumptions that do not fit venv's structure. The
functionality is implemented, but the check is logically incompatible
with how venv does things.

@skip_venv_site_packages
Same as above, but specifically this has something to do with venv's
central configuration file, and can be fixed by implementing a parallel
test.

@skip_venv_cp
This uses the "pew cp" command, which is not implemented yet. I will try
to eliminate them if possible.

This provide a more hard-nose effort to find a real Python in all places
possible. Should work if you install Python from the standard CPython
distribution. If not, I don't know what will.

Add the write permission to all activate scripts before copying the
venv if needed, and restore all modified files after cloning.

Why this is needed:
#173 (comment)