Enhancement: Upgrade Sphinx version to eliminate incorrect syntax errors

[pushfoo]

Older Sphinx versions, including pyglet's, do not report correct locations for syntax errors. Upgrading to 6.2.1 or later should fix some of the worst of these (see sphinx-doc/sphinx#11026).

Why this is relevant

1. It's annoying when working on doc
2. It has been discussed without objections before on Discord
3. It delays PRs when people unfamiliar with the problem run into it (see this example from arcade)

For an example of how this affects our current code:

/home/pushfoo/src/pyglet/pyglet/graphics/shader.py:docstring of pyglet.graphics.shader.Attribute:1: WARNING: Definition list ends without a blank line; unexpected unindent.

However, there is no definition list at line 1 of Attribute's docstring:

pyglet/pyglet/graphics/shader.py

Lines 151 to 153 in 66466f8

	class Attribute:
	"""Abstract accessor for an attribute in a mapped buffer."""

Tasks required

To my understanding per prior discussion on Discord:

• We're still on an old version because of our readthedocs config
• Benjamin has no objections to upgrading as long as build keeps working

This implies we have to update the following:

• Our sphinx version
  • Important note: Sphinx 7.2.0+ requires Python 3.9+, which can create an ugly situation for listing project requirements + readthedocs
• Any incompatible plugins
• Our readthedocs config
• Any build scripts which need version bumps (I can't find any yet)

[pushfoo]

Another benefit of using Sphinx 7.1+ is that we can show the full copyright list from LICENSE:

Changed in version 7.1: The value may now be a sequence of copyright statements in the above form, which will be displayed each to their own line.

[pushfoo]

Closing per Discord discussion and db94ffd