fix(strutils): stop singularize() mangling words ending in 'ss'

[Sanjays2402]

Summary

strutils.singularize() corrupts words that are already singular and end in a double s (glass, boss, class, kiss, address, business, ...), returning glas, bos, clas, kis, addres, busines.

>>> from boltons.strutils import singularize
>>> singularize('glass')
'glas'      # expected 'glass'
>>> singularize('boss')
'bos'       # expected 'boss'
>>> singularize('address')
'addres'    # expected 'address'

Root cause

singularize() ends with a catch-all that strips the final character from any word ending in s that didn't match the -ies / -ses branches:

elif word.endswith('es') and word[-3] == 's':
    singular = word[:-2]
else:
    singular = word[:-1]      # <-- 'glass' -> 'glas'

No English plural ends in ss. The plural of these words ends in sses (glass -> glasses), which is already handled correctly by the preceding word.endswith('es') and word[-3] == 's' branch. Only words that are already singular and end in ss fall through to the blind word[:-1].

This also violated the idempotency implied by the function's own docstring (singularize('Glasses') == 'Glass'): feeding that canonical singular back in produced 'Glas', and applying singularize() defensively to an already-singular word degraded it further ('boss' -> 'bos' -> 'bo').

Fix

Add an elif word.endswith('ss') guard before the final fallback that returns the word unchanged (+7 lines). Because the -sses branch runs first, real plurals are unaffected.

Before / after

input	before	after
glass	glas	glass
boss	bos	boss
class	clas	class
kiss	kis	kiss
address	addres	address
business	busines	business
glasses (plural)	glass	glass (unchanged)
bosses (plural)	boss	boss (unchanged)
chances, cats, activities	correct	correct (unchanged)

The single-s ambiguous cases (e.g. bus) are intentionally left alone — they are indistinguishable from a regular plural (cats -> cat) without a dictionary, so this PR does not touch them.

Tests

Added test_singularize_double_s covering the double-s singular words, case preservation (Glass, BOSS), the still-correct -sses plurals, and idempotency.

Proven to guard the bug (regression test fails on unpatched source, passes with the fix):

# source fix stashed, test kept:
>   assert singularize('glass') == 'glass'
E   AssertionError: assert 'glas' == 'glass'
1 failed

# fix restored:
1 passed

Full suite: 438 passed (was 437). All 28 strutils docstring doctests pass. No linter is configured in CI; the change is pure stdlib control flow valid on every supported Python (3.7-3.14).