tools/mpremote: Reduce dependency on importlib_metadata.

No longer require importlib_metadata on new Python versions as it is
included in the standard distribution.

Signed-off-by: Jochen Sprickerhof <git@jochen.sprickerhof.de>

tools/mpremote/requirements.txt

@@ -1,2 +1,2 @@
 pyserial >= 3.3
-importlib_metadata >= 1.4
+importlib_metadata >= 1.4; python_version < "3.8"