DEP: drop support for Python 3.9 #1856

Job	Run time
type-checking (3.10)	20s
type-checking (3.11)	19s
type-checking (3.12)	28s
Image tests	18s
type-checking (3.13)	22s
ubuntu-latest-py3.10	46s
ubuntu-latest-py3.11	51s
ubuntu-latest-py3.12	1m 3s
ubuntu-latest-py3.13	1m 0s
ubuntu-20.04-py3.10 (oldest deps, mininmal)	58s
ubuntu-20.04-py3.10 (oldest deps, full)	1m 3s
ubuntu-latest-py3.13 (full)	1m 6s
macos-latest-py3.13	57s
windows-latest-py3.13	1m 34s
docs	13s
Combine coverage reports	15s
	11m 33s

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