Update pulp-python docs links
The readme points to https://pulp-python.readthedocs.io/en/3.0-dev/
This versions includes a note: "You are not reading the most recent version of this documentation. 3.0.0b1 is the latest version available."
3.0.0b1 links to https://pulp-python.readthedocs.io/en/3.0.0b1/release-notes/3.0.z/, which is a broken link, and its not actually the latest release.
In the bottom left "versions" bar, it shows:
I think we probably ought to remove 3.0-dev. "latest" points to 3.0.0b3, which is correct, but IMO that should be explicitly listed here as well.