doc: mention cc-ing nodejs/python team for reviews

Add the nodejs/python github team to the table of people to /cc for
reviews on python code.

PR-URL: nodejs#10637
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>

doc/onboarding-extras.md

@@ -30,6 +30,7 @@
 | upgrading c-ares | @jbergstroem |
 | upgrading http-parser | @jbergstroem, @nodejs/http |
 | upgrading libuv | @saghul |
+| python code | @nodejs/python |
 | platform specific | @nodejs/platform-{aix,arm,freebsd,macos,ppc,smartos,s390,windows} |