Add type annotations, minor cleanups #3632

Job	Run time
Check towncrier / Towncrier Checks	36s
Pre-commit checks / Pre-commit Checks	1m 5s
Python Package (cocoa) / Create Python Package	1m 20s
Python Package (android) / Create Python Package	1m 41s
Python Package (core) / Create Python Package	1m 17s
Python Package (demo) / Create Python Package	1m 16s
Python Package (dummy) / Create Python Package	1m 8s
Python Package (gtk) / Create Python Package	1m 24s
Python Package (iOS) / Create Python Package	1m 23s
Python Package (toga) / Create Python Package	1m 25s
Python Package (web) / Create Python Package	1m 16s
Python Package (winforms) / Create Python Package	1m 9s
core (macos, 3.8)	2m 14s
core (macos, 3.9)	53s
core (macos, 3.10)	49s
core (macos, 3.11)	53s
core (macos, 3.12-dev)	2m 28s
core (ubuntu, 3.8)	48s
core (ubuntu, 3.9)	1m 10s
core (ubuntu, 3.10)	41s
core (ubuntu, 3.11)	44s
core (ubuntu, 3.12-dev)	1m 41s
core (windows, 3.8)	1m 15s
core (windows, 3.9)	1m 19s
core (windows, 3.10)	1m 20s
core (windows, 3.11)	1m 32s
core (windows, 3.12-dev)	3m 17s
Combine & check core coverage.	45s
backend (android)	35s
backend (cocoa)	51s
backend (gtk)	3m 6s
backend (iOS)	48s
backend (web)	44s
backend (winforms)	1m 4s
testbed (macOS)	2m 7s
testbed (windows)	3m 22s
testbed (linux)	3m 30s
testbed (android)	23m 13s
testbed (iOS)	6m 36s
	1h 22m 45s

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