Add us.states.enumeration() returning a dynamic Enum

[jcarbaugh]

Summary

Adds a new us.states.enumeration() helper that dynamically constructs an Enum of states keyed by state abbreviation. It mirrors the call shape of us.states.mapping(): an optional states iterable selects which State objects to include (defaulting to STATES_AND_TERRITORIES), and a value_field argument picks which State attribute supplies each member's value.

The enum member name is always the state's abbr — abbr is guaranteed to be a valid Python identifier, so users can't accidentally produce invalid member names by picking a value_field like "name" ("New York") or "fips" ("01").

Closes #28

Example

>>> States = us.states.enumeration()
>>> States.VA
<States.VA: 'Virginia'>

>>> States = us.states.enumeration("fips")
>>> States.CA.value
'06'

>>> States = us.states.enumeration("fips", states=[us.states.DC, us.states.MD])
>>> list(States)
[<States.DC: '11'>, <States.MD: '24'>]

Approach

enumeration() lives next to mapping() in us/states.py and delegates to the stdlib functional API: Enum("States", {s.abbr: getattr(s, value_field) for s in states}). No changes to mapping() or to State.

Test plan

• uv run pytest us/tests/test_us.py passes
• Default value_field ("name") returns expected member values
• Explicit states= iterable produces an enum of exactly that subset
• Non-name value_field ("fips") values pass through unchanged

[jcarbaugh]

Review notes:

• us/states.py:207-210 — new enumeration() function. The key design choice is hard-coding the member name as state.abbr (always a valid Python identifier) and only letting the caller pick the value via value_field. Worth a sanity check that this is the right ergonomic tradeoff vs. exposing a name_field too.
• Return type is Type[Enum] — the function returns a dynamically constructed Enum subclass, not an instance.
• Edge case to consider: passing states=us.STATES_AND_TERRITORIES + us.OBSOLETE with value_field="fips" would produce multiple None values, which Enum would alias. Default state set excludes obsolete states so this only bites callers who opt in.
• No new exports. Same access pattern as mapping() — reached via us.states.enumeration.
• README has a new "Enumerations" section; CHANGELOG has a single bullet under the unreleased entry.