Fix pandas version check, require Python ≥ 3.8

[kddubey]

Hello,

This corrects the version check in #105. The current check will fail when we're (hopefully) retired and pandas v10 comes out:

print("10.0.0" < "2.1.0")
# True
# yikes!

from packaging.version import Version
print(Version("10.0.0") < Version("2.1.0"))
# False

Docs for packaging.version.Version

The PR adds a new required dependency from PyPA: packaging. It has no dependencies but requires Python ≥ 3.8. I'm not sure if this change is too breaking b/c the test workflow only tests Python ≥ 3.9, and bambi requires Python ≥ 3.10. And Python 3.7 reached EOL on 2023-06-27. Feel free to close the PR if this breaking change is too much breaking / you want to fix the version check a different way

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.63%. Comparing base (b00f53d) to head (fb320c3).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #110   +/-   ##
=======================================
  Coverage   92.63%   92.63%           
=======================================
  Files          20       20           
  Lines        2227     2228    +1     
=======================================
+ Hits         2063     2064    +1     
  Misses        164      164           

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[tomicapretto]

@kddubey thanks for the contribution and sorry for taking so long to review and merge it.