Include python 3.15 in test matrix

[akaIDIOT]

No description provided.

[github-actions]

Comparing median (lower is better) metric of benchmarks between this PR's target (96758d6) and the HEAD of this PR (a23ac12):

	3.10	3.11	3.12	3.13	3.14	3.15	PyPy 3.10	PyPy 3.11
test_benchmark_get	+1%	+1%	-0%	-4%	+3%	+2%	+3%	+7%
test_benchmark_get_dotted	-0%	+2%	+1%	-2%	-0%	-4%	-2%	-1%
test_benchmark_getattr	-1%	-0%	-0%	+4%	+3%	+2%	+1%	+0%
test_benchmark_getitem	-2%	-3%	+1%	-3%	+4%	+1%	+1%	+4%
test_benchmark_getitem_dotted	-3%	+3%	+2%	-8%	-2%	-1%	-1%	+1%
test_benchmark_init_full_overlap	-0%	+1%	-1%	-4%	+0%	+5%	-0%	+1%
test_benchmark_init_no_overlap	-1%	+2%	+0%	-7%	-1%	+3%	+0%	+1%
test_benchmark_init_partial_overlap	-0%	+0%	-1%	-5%	-1%	+4%	+0%	+1%
test_benchmark_match_mapping	-0%	+1%	-1%	+3%	+1%	+2%	-1%	+2%
test_benchmark_reference_chain	+0%	+3%	-0%	-2%	-1%	+1%	-0%	+5%
test_benchmark_splat_args	+1%	+3%	-0%	-1%	+0%	-2%	+1%	+0%
test_benchmark_spread_kwargs	-3%	+4%	+0%	+4%	+0%	+5%	+1%	-2%

(This comment will be updated on subsequent pushes)