Skip installation for tox jobs that don't need the in-developement pip

tox.ini

@@ -19,6 +19,7 @@ basepython = python3
 commands = pytest --timeout 300 --cov=pip --cov-report=term-missing --cov-report=xml --cov-report=html tests/unit {posargs}
 
 [testenv:docs]
+# Don't skip install here since docs.pipext uses pip's internals.
 deps = -r{toxinidir}/tools/docs-requirements.txt
 basepython = python2.7
 commands =
@@ -26,6 +27,7 @@ commands =
     sphinx-build -W -d {envtmpdir}/doctrees -b man  docs docs/build/man
 
 [testenv:packaging]
+skip_install = True
 deps =
     check-manifest
     readme_renderer
@@ -40,16 +42,19 @@ commands =
     isort --check-only --diff
 
 [testenv:lint-py2]
+skip_install = True
 basepython = python2
 deps = {[lint]deps}
 commands = {[lint]commands}
 
 [testenv:lint-py3]
+skip_install = True
 basepython = python3
 deps = {[lint]deps}
 commands = {[lint]commands}
 
 [testenv:mypy]
+skip_install = True
 basepython = python3
 deps = mypy
 commands =