Use `strict = True` for `mypy_bootstrap.ini` #13798

Job	Run time
Test mypyc suite with 32-bit Python	4m 7s
Test suite with py38-ubuntu, mypyc-compiled	1m 24s
Test suite with py38-windows-64	3m 58s
Test suite with py39-ubuntu	4m 8s
Test suite with py310-ubuntu	4m 9s
Test suite with py311-ubuntu, mypyc-compiled	1m 12s
Test suite with py312-ubuntu, mypyc-compiled	1m 5s
Test suite with py313-ubuntu, mypyc-compiled	1m 12s
mypyc runtime tests with py39-macos	4m 6s
Type check our own code (py38-ubuntu)	1m 55s
Type check our own code (py38-windows-64)	2m 24s
Formatting and code style with Black + ruff	53s
	30m 33s

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