Allow label for SP800_108_Counter to include null bytes #775

Job	Run time
build_python_27 (ubuntu-20.04, yes)	2m 15s
build_python_27 (ubuntu-20.04, no)	2m 17s
mypy	18s
build_python_27 (windows-latest, yes)	3m 49s
build (3.5, yes, ubuntu-20.04)	2m 14s
build_python_27 (windows-latest, no)	3m 46s
build (3.5, no, ubuntu-20.04)	2m 3s
test_c_linux (0, x64)	1m 36s
build (3.6, yes, ubuntu-20.04)	2m 7s
test_c_linux (0, x32)	1m 8s
test_c_windows (x64)	54s
build (3.6, no, ubuntu-20.04)	2m 50s
test_c_linux (1, x64)	1m 9s
test_c_windows (win32)	1m 36s
build (3.7, yes, ubuntu-20.04)	3m 20s
test_c_linux (1, x32)	1m 27s
build (3.7, no, ubuntu-20.04)	2m 13s
build (3.8, yes, ubuntu-20.04)	1m 54s
build (3.8, no, ubuntu-20.04)	3m 1s
build (3.9, yes, ubuntu-20.04)	2m 46s
build (3.9, no, ubuntu-20.04)	1m 56s
build (3.10, yes, ubuntu-20.04)	1m 53s
build (3.10, no, ubuntu-20.04)	1m 54s
build (3.11, yes, ubuntu-20.04)	1m 39s
build (3.11, no, ubuntu-20.04)	1m 42s
build (3.12-dev, yes, ubuntu-20.04)	1m 48s
build (3.12-dev, no, ubuntu-20.04)	2m 44s
build (3.11, yes, macos-12)	3m 35s
build (3.11, no, windows-latest)	5m 18s
build (3.11, yes, windows-latest)	4m 21s
build (3.12-dev, no, windows-latest)	4m 14s
build (3.12-dev, yes, windows-latest)	3m 37s
build (pypy2.7, no, ubuntu-latest)	1m 24s
build (pypy3.9, no, ubuntu-latest)	2m 28s
	1h 21m 16s

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