update python client's CHANGELOG

[melange396]

The CHANGELOG.md file for the python client is out of date.

As of this issue's creation (10 May 2024), it is missing:

• for version 4.1.13:
  • covidcast_nowcast endpoint removal #1323
  • Prevent "308 PERMANENT REDIRECT" via client change #1330
• for version 4.1.17:
  • Replace deprecated pkg_resources #1363
• staged for release (likely to be in upcoming version 4.1.21):
  • Add inspection modes to Python client library #1418

We should also update the reference to the changelog in the package metadata so that it points to the main branch instead of dev