tools/mpremote: Allow running mpremote with python -m.

This is helpful because some scripts are likely to use mpremote with a
specific python path.

tools/mpremote/mpremote/__main__.py

@@ -0,0 +1,6 @@
+#!/usr/bin/env python3
+
+import sys
+from mpremote import main
+
+sys.exit(main.main())