Fix variance issue in `_IncEx` type alias, only allow `True` #5064

Job	Run time
coverage	2m 51s
test 3.8	2m 9s
test on ubuntu	1m 58s
test 3.9	2m 9s
test on macos	1m 52s
test 3.10	1m 54s
test on windows	3m 51s
rust benchmarks	2m 43s
test 3.11	2m 25s
build sdist	14s
test 3.12	1m 58s
test 3.13	2m 0s
build-wasm-emscripten	2m 35s
test pypy3.9	2m 26s
lint	58s
test pypy3.10	2m 11s
test-debug 3.11	59s
test-debug pypy3.10	1m 14s
test MSRV	2m 21s
test-pydantic-integration	1m 56s
build on ${{ matrix.os }} (${{ matrix.target }} - ${{ matrix.interpreter \|\| 'all' }}${{ (((matrix.os == 'linux') && format(' - {0}', (((matrix.manylinux == 'auto') && 'manylinux') \|\| matrix.manylinux))) \|\| '') }})	0s
build pgo-optimized on ${{ matrix.os }} / ${{ matrix.interpreter }}	0s
test build on ${{ matrix.os }}	0s
test build on ${{ matrix.target }}-${{ matrix.distro }}	0s
inspect-pypi-assets	0s
check	5s
release	0s
	40m 49s

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