Replace inspect usage with common CPython stdlib pattern

[Sachaa-Thanasius]

Part of #326.

This is the biggest source of import time and hopefully the least controversial to replace; based on local testing, replacing this improves it by ~30% on supported CPython versions and PyPy3.10.

This replaces the expensive inspect import and inspect.stack() usage with an expanded version of a frame inspection pattern used in the CPython standard library.

It's a bit more code, but it's a fairly reasonable amount considering it does the following:

1. Has various fast paths.
2. Chooses the fastest working method of inspection at first call to avoid needing to choose again in the future.
3. Provides robust support for several alternate Python implementations that don't have many, if any, accessible stack frames.
4. Obviously, avoids an expensive import.

References:

• https://github.com/python/cpython/blob/e2476398ee9911b6b0b80e3ca182647805fde81f/Lib/enum.py#L871-L879
• https://github.com/python/cpython/blob/e2476398ee9911b6b0b80e3ca182647805fde81f/Lib/doctest.py#L228-L234
• https://github.com/python/cpython/blob/e2476398ee9911b6b0b80e3ca182647805fde81f/Lib/dataclasses.py#L1625-L1632
• https://github.com/python/cpython/blob/e2476398ee9911b6b0b80e3ca182647805fde81f/Lib/typing.py#L1840-L1849
• https://github.com/taleinat/python-stdlib-sentinels/blob/410273e40bdbd1b58a63cebefdf8abcce6c620e3/sentinels/sentinels.py#L95-L124

[Sachaa-Thanasius]

This can be slimmed down (as evidenced by the stdlib's version), but I figured I'd start with the most robust version.