Typo in __repr__ of SimpleTZ

[Ephtolens]

Hi !
Pb in this line ->

gpxpy/gpxpy/gpxfield.py

Line 74 in cad36d5

return 'SimpleTZ({self.tzname(None}!r)'

Should be f'SimpleTZ({self.tzname(None)}!r)' instead of 'SimpleTZ({self.tzname(None}!r)' I think

[tkrajina]

Fixed, thanks