run_mypy: accept the path to an alternative mypy executable

mypy might be installed with a different python than the one run_mypy.py
is using.

run_mypy.py

@@ -96,6 +96,7 @@ def main() -> int:
 
     parser = argparse.ArgumentParser(description='Process some integers.')
     parser.add_argument('files', nargs='*')
+    parser.add_argument('--mypy', help='path to mypy executable')
     parser.add_argument('-q', '--quiet', action='store_true', help='do not print informational messages')
     parser.add_argument('-p', '--pretty', action='store_true', help='pretty print mypy errors')
     parser.add_argument('-C', '--clear', action='store_true', help='clear the terminal before running mypy')
@@ -121,12 +122,10 @@ def main() -> int:
         to_check.extend(modules)
 
     if to_check:
+        command = [opts.mypy] if opts.mypy else [sys.executable, '-m', 'mypy']
         if not opts.quiet:
             print('Running mypy (this can take some time) ...')
-        p = subprocess.run(
-            [sys.executable, '-m', 'mypy'] + args + to_check,
-            cwd=root,
-        )
+        p = subprocess.run(command + args + to_check, cwd=root)
         return p.returncode
     else:
         if not opts.quiet: