Skip install on tox pylint (#39260)

We're not operating on the installed package anyway, and necessary
dependencies are handled with tox deps.

As a nice bonus side effect, doing this sidesteps breakage caused by
pip's (up to 20.2.2 at least) behavior of prepending site-packages to
sys.path in certain cases, which in turn results in failures e.g. if a
version of typing that is incompatible with the (now overridden)
stdlib is installed there. And that combined with also pip's behavior
of installing a default build system consisting of setuptools and
wheel under the hood when it sees our pyproject.toml without a
build-system defined would provoke the breakage before we have a
chance to uninstall typing. (There are ways around this too, but
skipping the install makes the issue moot at least with our current
dependency set.)

tox.ini

@@ -18,6 +18,7 @@ deps =
      -r{toxinidir}/requirements_test_all.txt
 
 [testenv:pylint]
+skip_install = True
 ignore_errors = True
 deps =
      -r{toxinidir}/requirements_all.txt