Allow overriding DEFAULT_MAX_THREADS on AvifImagePlugin module #438

Job	Run time
MinGW	6m 16s
ubuntu-20.04 Python pypy-3.7 (libavif 0.11.0)	1m 38s
ubuntu-20.04 Python 3.12 (libavif 0.11.0)	43s
ubuntu-20.04 Python 3.11 (libavif 0.11.0)	31s
ubuntu-20.04 Python 3.10 (libavif 0.11.0)	47s
ubuntu-20.04 Python 3.9 (libavif 0.11.0)	34s
ubuntu-20.04 Python 3.8 (libavif 0.11.0)	38s
ubuntu-20.04 Python 3.7 (libavif 0.11.0)	38s
ubuntu-20.04 Python 2.7 (libavif 0.11.0)	45s
macos-13 Python pypy-3.7 (libavif 0.11.0)	35s
macos-13 Python 3.12 (libavif 0.11.0)	45s
macos-13 Python 3.11 (libavif 0.11.0)	55s
macos-13 Python 3.10 (libavif 0.11.0)	51s
macos-13 Python 3.9 (libavif 0.11.0)	39s
macos-13 Python 3.8 (libavif 0.11.0)	59s
macos-13 Python 3.7 (libavif 0.11.0)	1m 32s
ubuntu-20.04 Python 3.9 (libavif 88d3dccda111f6ccbcccd925179f67e7d6fdf4ff)	45s
Test Successful	1s
	19m 32s

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