PYTHON-5508 - Add built-in DecimalEncoder and DecimalDecoder #8037

Job	Run time
static	1m 33s
CPython 3.13t-ubuntu-latest	8m 46s
CPython 3.9-ubuntu-latest	8m 52s
CPython pypy-3.10-ubuntu-latest	8m 47s
Link Check	1m 42s
Docs Checks	41s
DocTest	2m 9s
Typing Tests (3.9)	28s
Test async's minimum dependencies and Python	48s
Typing Tests (3.11)	31s
Test using minimum dependencies and supported Python	39s
Make an sdist	14s
Install from SDist and Test	1m 23s
	36m 33s

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