tweak workaround for pypa#1644

Work around buggy pip detection code for "pip.exe install/update pip ...".

tools/tox_pip.py

@@ -17,12 +17,21 @@ def pip(args):
                                'pip'])
         shutil.rmtree(glob(os.path.join(TOX_PIP_DIR, 'pip-*.dist-info'))[0])
     # And use that version.
+    pypath = os.environ.get('PYTHONPATH')
+    pypath = pypath.split(os.pathsep) if pypath is not None else []
+    pypath.insert(0, TOX_PIP_DIR)
+    os.environ['PYTHONPATH'] = os.pathsep.join(pypath)
+    # Disable PEP 517 support when using editable installs.
     for n, a in enumerate(args):
         if not a.startswith('-'):
             if a in 'install' and '-e' in args[n:]:
                 args.insert(n + 1, '--no-use-pep517')
             break
-    subprocess.check_call([sys.executable, os.path.join(TOX_PIP_DIR, 'pip')] + args)
+    # Fix call for setuptools editable install.
+    for n, a in enumerate(args):
+        if a == '.':
+            args[n] = os.getcwd()
+    subprocess.check_call([sys.executable, '-m', 'pip'] + args, cwd=TOX_PIP_DIR)
 
 
 if __name__ == '__main__':

tox.ini

@@ -14,7 +14,7 @@ envlist=python
 pip = python {toxinidir}/tools/tox_pip.py
 
 [testenv]
-deps=-rtests/requirements.txt
+deps=-r{toxinidir}/tests/requirements.txt
 install_command = {[helpers]pip} install {opts} {packages}
 list_dependencies_command = {[helpers]pip} freeze --all
 setenv=COVERAGE_FILE={toxworkdir}/.coverage.{envname}