tweak workaround for pypa#1644

Work around buggy pip detection code for "pip.exe install/update ...  pip ...".
commit 0d831c9
Author: Benoit Pierre <benoit.pierre@gmail.com>
Date:   Sat Aug 10 03:57:58 2019 +0200

    improve

tools/tox_pip.py

@@ -17,12 +17,16 @@ 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)
     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)
+    subprocess.check_call([sys.executable, '-m', 'pip'] + args)
 
 
 if __name__ == '__main__':