Replace deprecated plot_date with plot

[m4cd4r4]

Fixes #603.

ax.plot_date() was deprecated in matplotlib 3.9. Replaced with ax.plot() in plot_airmass, plot_altitude, and plot_parallactic. Added ax.xaxis_date() before each formatting block so the axis still uses date-aware locating.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.28%. Comparing base (a4d89a0) to head (52e81b5).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #636      +/-   ##
==========================================
+ Coverage   72.90%   76.28%   +3.37%     
==========================================
  Files          14       14              
  Lines        1849     1851       +2     
==========================================
+ Hits         1348     1412      +64     
+ Misses        501      439      -62     

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[m4cd4r4]

Pushed a fix for the CI and RTD failures. The root cause was fmt being passed as a keyword argument to ax.plot() via style_kwargs, which newer matplotlib no longer accepts - fmt is a positional argument, not a Line2D property.

Changed all style_kwargs.setdefault('fmt', '-') calls to style_kwargs.setdefault('linestyle', '-'), and cleaned up the redundant guard checks that referenced fmt.