wxui: support python 3.9

I get the following exception on start:

$ python3 chirpwx.py
Traceback (most recent call last):
File "/home/asheplyakov/work/radio/chirp/chirpwx.py", line 8, in
<module>
    sys.exit(chirpmain())
File "/home/asheplyakov/work/radio/chirp/chirp/wxui/__init__.py", line
41, in chirpmain
    from chirp.wxui import main
File "/home/asheplyakov/work/radio/chirp/chirp/wxui/main.py", line 28,
in <module>
    import importlib_resources
ModuleNotFoundError: No module named 'importlib_resources'

$ python3 --version
Python 3.9.6

However importlib is available in Python 3.9 (as a part of
the standard library). To avoid the problem this patch removes
the version check, and checks for
importlib_resources/importlib.resources modules instead.

chirp/wxui/__init__.py

@@ -5,9 +5,9 @@
 import os
 import sys
 
-if sys.version_info < (3, 10):
+try:
     import importlib_resources
-else:
+except ModuleNotFoundError:
     import importlib.resources as importlib_resources
 
 from chirp import directory

chirp/wxui/main.py

@@ -24,9 +24,9 @@
 import time
 import webbrowser
 
-if sys.version_info < (3, 10):
+try:
     import importlib_resources
-else:
+except ModuleNotFoundError:
     import importlib.resources as importlib_resources
 
 import wx

setup.py

@@ -15,7 +15,7 @@
           'pyserial',
           'six',
           'future',
-          'importlib-resources;python_version<"3.10"',
+          'importlib-resources;python_version<"3.9"',
           'yattag',
       ],
       extras_require={