Make FontFamilyDropdown compatible w/ Python 3.13

[JohnStrunk]

Several of the widgets are incompatible w/ Python 3.13 due to the use of is instead of ==. This PR fixes those instances that interfere with the usage of the FontFamilyDropdown widget.

[codecov[bot]]

Codecov Report

:x: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review. :white_check_mark: Project coverage is 89.48%. Comparing base (ebaa7d4) to head (51694a6).

Files with missing lines	Patch %	Lines
ttkwidgets/font/familydropdown.py	50.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #109      +/-   ##
==========================================
- Coverage   89.53%   89.48%   -0.06%     
==========================================
  Files          43       43              
  Lines        4032     4021      -11     
==========================================
- Hits         3610     3598      -12     
- Misses        422      423       +1     

Files with missing lines	Coverage Δ
ttkwidgets/font/familylistbox.py	95.65% <100.00%> (-0.19%)	:arrow_down:
ttkwidgets/font/sizedropdown.py	94.11% <100.00%> (-0.33%)	:arrow_down:
ttkwidgets/font/familydropdown.py	73.68% <50.00%> (-1.32%)	:arrow_down:

... and 5 files with indirect coverage changes

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