TypeError in gpxfield since 1.6.0 release

[ThomasKalverda]

Weirdly enough, since the 1.6.0 release a couple of hours ago, I also get this error when I pin the version on 1.5.0 🤷

  ...
    import gpxpy
  File "/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/site-packages/gpxpy/__init__.py", line 17, in <module>
    from . import gpx as mod_gpx
  File "/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/site-packages/gpxpy/gpx.py", line 27, in <module>
    from . import gpxfield as mod_gpxfield
  File "/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/site-packages/gpxpy/gpxfield.py", line 36, in <module>
    class SimpleTZ(mod_datetime.tzinfo):
  File "/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/site-packages/gpxpy/gpxfield.py", line 70, in SimpleTZ
    def __deepcopy__(self, memodict: Optional[dict[Any, Any]]={}) -> mod_datetime.tzinfo:
TypeError: 'type' object is not subscriptable

[doronz88]

I created a fix in #268
please merge and release a new version since it currently breaks my package