Exclude the physical-type aliases from the default numpydoc xref aliases

[astrofrog]

This is to fix astropy issue #19868: in the
rendered simple_norm docs, the 'power' option of stretch : {'linear', 'sqrt', 'power', ...} is turned into a hyperlink to the power physical type. It's a meaningless link — 'power' there is a literal option value, not a unit physical type.

This happens because sphinx_astropy/conf/v2.py builds numpydoc_xref_aliases_astropy_physical_type with keys that are quoted strings ('power', 'energy', 'time', ...) and folds them into the default numpydoc_xref_astropy_aliases.

Numpydoc gives a quoted string a single meaning regardless of context, but the
common context is an enum of literal options:

stretch : {'linear', 'sqrt', 'power', ...}

Here 'power' is a literal choice, not a type — but it's the same token the alias is keyed on, so alias substitution rewrites it into a cross-reference. About a dozen physical-type names collide with ordinary option words (area,
energy, force, length, mass, power, pressure, speed, time, volume, ...), so any {...} enum using one of those words gets a spurious link.

These aliases were added as an opt-in in #40. The intended use was a parameter whose type is a physical quantity, written as a quoted name — e.g. length : 'length' linking to astropy's physical-types docs. The original comment documents them as something packages turn on explicitly with numpydoc_xref_aliases.update(numpydoc_xref_astropy_aliases) — but glossary and physical-type aliases were fused into a single ChainMap, so a package couldn't take the safe glossary aliases without also pulling in the colliding physical-type ones.

They also appear never to have actually worked properly anyway: the alias value has a double :ref: (:ref::ref: '{ptype}' <...>``) since the very first commit (this is also fixed here)

The fix is to drop numpydoc_xref_aliases_astropy_physical_type from the default numpydoc_xref_astropy_aliases ChainMap. It stays defined and documented as an explicit opt-in for packages that genuinely want physical-type cross-refs (having it opt-in was the original intent):

numpydoc_xref_aliases.update(numpydoc_xref_aliases_astropy_physical_type)

The glossary aliases remain on by default — they're all hyphenated -like terms (time-like, etc.) that don't collide with literal option values, so they're safe(r).

This also fixes the double :ref:, so the opt-in path now renders a correct link instead of literal :ref: 'power' text.

In future we could consider not automagically changing 'power' to a ref but having some kind of namespace like physical_type:power in docstrings to avoid any ambiguities.

[pllim]

Thank you!

[pllim]

At least for the problem I saw, this PR seems to fix it. Yay!

astropy/astropy#19891