DOC/CI: fix doc build with Python 3.9

[jorisvandenbossche]

The doc build was failing on master (/home/runner/work/pandas/pandas/doc/source/reference/api/pandas.Series.dt.rst: WARNING: document isn't included in any toctree), which started when the environment switched to Python 3.9.

It is now pinned to Python 3.8 (#41092), but we should fix the warning for Python 3.9, and unpin the environment.

cc @pandas-dev/pandas-docs

[simonjayhawkins]

@jorisvandenbossche changed the milestone since we want to do this before the 1.2.5 release.

[jorisvandenbossche]

To be clear, the docs are still building fine now. I think it's OK if we need to keep the Python 3.8 pin on the 1.2.x branch?

[simonjayhawkins]

The thing is, since we are changing environment.yml just for ci, do we want that change in an actual release?

[jorisvandenbossche]

How does the environment.yml impact the release? (eg the file is not included in the sdist)

[simonjayhawkins]

eg the file is not included in the sdist

didn't know that.

[simonjayhawkins]

eg the file is not included in the sdist

didn't know that.

[simonjayhawkins]

eg the file is not included in the sdist

didn't know that.

removing the milestone