Update implementation details to be compatible with mypy2

[akaIDIOT]

Using type() to create NotConfigured was done for mypy, which broke for mypy2. Using a proper class helps. This PR also ensures NotConfigured can be pickled and comes back as the same singleton-like instances.

[github-actions]

Comparing median (lower is better) metric of benchmarks between this PR's target (63b97c5) and the HEAD of this PR (5dc48e7):

	3.10	3.11	3.12	3.13	3.14	PyPy 3.10	PyPy 3.11
test_benchmark_get	-1%	+4%	-5%	-2%	+2%	-4%	+2%
test_benchmark_get_dotted	+2%	+3%	-4%	-3%	+5%	+5%	+2%
test_benchmark_getattr	-1%	-1%	-1%	-4%	-0%	-5%	-1%
test_benchmark_getitem	+2%	+4%	-4%	-1%	+3%	-3%	-3%
test_benchmark_getitem_dotted	-1%	+5%	-1%	-4%	+6%	+2%	+2%
test_benchmark_init_full_overlap	+2%	+1%	-6%	+1%	+0%	-2%	-2%
test_benchmark_init_no_overlap	+3%	+1%	-5%	-3%	+2%	-4%	-1%
test_benchmark_init_partial_overlap	+3%	+2%	-7%	-3%	-1%	-3%	-0%
test_benchmark_reference_chain	-2%	+2%	-2%	+2%	-1%	+2%	-1%
test_benchmark_splat_args	+1%	+2%	+1%	+3%	+1%	+1%	-5%
test_benchmark_spread_kwargs	-1%	+2%	-2%	-2%	+2%	+2%	-1%

(This comment will be updated on subsequent pushes)