Extract pip wrapper launcher. Ref #1644.

pip-wrapper.py

@@ -0,0 +1,18 @@
+"""
+Wrap the pip command to:
+
+- Avoid the default 'python -m pip' invocation, which causes the current
+  working directory to be added to the path, which causes problems.
+"""
+
+
+import sys
+import subprocess
+
+
+def main():
+    cmd = sys.argv[1:]
+    subprocess.check_call(cmd)
+
+
+__name__ == '__main__' and main()

tox.ini

@@ -9,12 +9,8 @@ envlist=python
 
 [testenv]
 deps=-rtests/requirements.txt
-# Changed from default (`python -m pip ...`)
-# to prevent the current working directory
-# from being added to `sys.path`.
-install_command={envbindir}/pip install {opts} {packages}
-# Same as above.
-list_dependencies_command={envbindir}/pip freeze
+install_command = python ./pip-wrapper.py {envbindir}/pip install {opts} {packages}
+list_dependencies_command = python ./pip-wrapper.py {envbindir}/pip freeze
 setenv=COVERAGE_FILE={toxworkdir}/.coverage.{envname}
 # TODO: The passed environment variables came from copying other tox.ini files
 # These should probably be individually annotated to explain what needs them.