Update Stackless additions to Doc/conf.py

[akruis]

Are theses lines still required and up to date?

stackless/Doc/conf.py

Lines 19 to 33 in a599283

	# Compatibility hack for Sphinx version 1.2 and later
	# without the 'sphinx.ext.refcounting' extension
	# Required for http://stackless.readthedocs.org
	try:
	# this symbol is available starting from version 1.2
	from sphinx import version_info
	except Exception:
	pass # version is pre 1.2.
	else:
	del version_info
	del extensions[0]
	if os.environ.get('READTHEDOCS', None) == 'True':
	# Use the Sphinx default theme, not the very neat theme of readthedocs.org
	# See https://docs.readthedocs.org/en/latest/faq.html#i-want-to-use-the-blue-default-sphinx-theme
	html_style = '/default.css'

Remove or update them.

[akruis]

Readthedocs is always a moving target, but now (commit 53ca885) the documentation for branch 3.8-slp builds correctly. See https://stackless.readthedocs.io/en/3.8-slp/stackless-python.html.

Building the Stackless documentation on readthedocs.org now involves 2 files only:

1. .readthedocs.yaml: overall specifications, refer to Doc/slp_readthedocs_requirements.txt
2. Doc/slp_readthedocs_requirements.txt: defines the virtual env used to run Sphinx. Must match the
   requirements in target venv of Doc/Makefile

These changes are now in branch main-slp and 3.8-slp.