Drop Python 3.7 support

[martinfleis]

Dropping Python 3.7, following NEP 29. I have also removed the old unused travis.yml.

edit: I have also changed the frequency of nightly CI builds to twice a week instead of daily.

[codecov]

Codecov Report

Merging #151 (33e30e5) into main (10fbce9) will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #151      +/-   ##
==========================================
+ Coverage   86.05%   86.12%   +0.06%     
==========================================
  Files          18       18              
  Lines        1226     1232       +6     
==========================================
+ Hits         1055     1061       +6     
  Misses        171      171              

Impacted Files	Coverage Δ
splot/_viz_esda_mpl.py	100.00% <0.00%> (ø)
splot/_viz_value_by_alpha_mpl.py	96.12% <0.00%> (+0.03%)	⬆️
splot/_viz_giddy_mpl.py	94.73% <0.00%> (+0.03%)	⬆️
splot/_viz_libpysal_mpl.py	98.14% <0.00%> (+0.03%)	⬆️
splot/_viz_utils.py	57.00% <0.00%> (+0.43%)	⬆️
splot/_viz_bokeh.py	17.30% <0.00%> (+0.80%)	⬆️

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