Lint with ruff

[jGaboardi]

This PR:

• Lints code base with ruff; I opened 3 issues that can be addressed later that were found by linting:
  • nan_dot is undefined #37
  • refactor utils.py – numpy<"1.7.1" no longer supported #38
  • unused lines of code in glm.py and varfuncs.py #41
• Adds a codecov badge to README.

[codecov]

Codecov Report

Merging #40 (b8ce111) into main (7cffe56) will decrease coverage by 0.11%.
The diff coverage is 58.33%.

@@            Coverage Diff             @@
##             main      #40      +/-   ##
==========================================
- Coverage   69.86%   69.76%   -0.11%     
==========================================
  Files           9        9              
  Lines         999      999              
==========================================
- Hits          698      697       -1     
- Misses        301      302       +1     

Files	Coverage Δ
spglm/__init__.py	100.00% <100.00%> (ø)
spglm/family.py	82.16% <100.00%> (ø)
spglm/glm.py	97.03% <100.00%> (ø)
spglm/iwls.py	82.45% <ø> (-0.31%)	⬇️
spglm/base.py	45.37% <0.00%> (ø)
spglm/utils.py	27.95% <50.00%> (ø)
spglm/varfuncs.py	77.35% <0.00%> (-1.49%)	⬇️