Pin `towncrier` in docs requirements #6930

Job	Run time
docs	41s
determine-changes	3s
packaging	31s
vendoring	0s
tests / ${{ matrix.python.key \|\| matrix.python }} / ${{ matrix.os }}	0s
tests / zipapp	0s
tests / ${{ matrix.python }} / ${{ matrix.os }} / ${{ matrix.group }}	0s
check	3s
	1m 18s

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