Workaround cgi removal in Python3.13

[mmore500]

Currently, importing ete3 in Python3.13 fails due to the standard library cgi module having been removed.

Python 3.13.1 (main, Dec  9 2024, 00:00:00) [GCC 14.2.1 20240912 (Red Hat 14.2.1-3)] on linux
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>>> import ete3
Traceback (most recent call last):
  File "<python-input-0>", line 1, in <module>
    import ete3
  File "/home/mmore500/.local/lib/python3.13/site-packages/ete3/__init__.py", line 56, in <module>
    from .webplugin.webapp import *
  File "/home/mmore500/.local/lib/python3.13/site-packages/ete3/webplugin/webapp.py", line 44, in <module>
    import cgi
ModuleNotFoundError: No module named 'cgi'

As a temporary solution, this pull request patches ete3/__init__.py to catch the generated ModuleNotFoundError and warn the user that webapp support is not available in Python3.13.