Fix `PyUnicode` functions for Python 3.13 #12876

Job	Run time
Test mypyc suite with 32-bit Python	6m 37s
Test suite with py38-ubuntu, mypyc-compiled	16m 31s
Test suite with Python nightly	12m 25s
Test suite with py38-windows-64	19m 33s
Test suite with py39-ubuntu	16m 18s
Test suite with py310-ubuntu	15m 47s
Test suite with py311-ubuntu, mypyc-compiled	15m 41s
Test suite with py312-ubuntu, mypyc-compiled	17m 19s
mypyc runtime tests with py39-macos	4m 59s
mypyc runtime tests with py38-debug-build-ubuntu	4m 21s
Type check our own code (py38-ubuntu)	1m 43s
Type check our own code (py38-windows-64)	2m 23s
Formatting and code style with Black + ruff	37s
	2h 14m 14s

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