Use getargspec only with Python 2, fix for Kodi 20

[antonsoroko]

Found small error in "compatibility" code: it used to work for python 3 since getargspec still existed until Python 3.11.
But Kodi 20 uses Python 3.11 - thus addon does not work anymore:

File "/storage/emulated/0/Android/data/org.xbmc.kodi/files/.kodi/addons/script.module.cherrypy/lib/cherrypy/_cpdispatch.py", line 209, in <module>
  getargspec = inspect.getargspec
               ^^^^^^^^^^^^^^^^^^
AttributeError: module 'inspect' has no attribute 'getargspec'

Fix tested on python 2 and 3.

https://docs.python.org/3/whatsnew/3.11.html#removed

https://docs.python.org/2/library/inspect.html#inspect.getargspec
https://docs.python.org/3/library/inspect.html#inspect.getfullargspec