Add mutation testing to CI #258

Job	Run time
test (py2.6, ubuntu-latest, centos:6, 2.6)	1m 56s
test (py3.10 with ossl3.0, ubuntu-latest, ubuntu:22.04, 3.10, py310)	2m 50s
test (py2.7, ubuntu-20.04, 2.7, py27)	2m 47s
test (py2.7 with old gmpy, ubuntu-20.04, 2.7, py27_old_gmpy)	1m 52s
test (py2.7 with old gmpy2, ubuntu-20.04, 2.7, py27_old_gmpy2)	2m 59s
test (py2.7 with old six, ubuntu-20.04, 2.7, py27_old_six)	3m 32s
test (py2.7 with gmpy, ubuntu-20.04, 2.7, gmpypy27)	3m 12s
test (py2.7 with gmpy2, ubuntu-20.04, 2.7, gmpy2py27)	2m 33s
test (py3.5, ubuntu-20.04, 3.5, py35)	2m 41s
test (py3.6, ubuntu-20.04, 3.6, py36)	2m 48s
test (py3.7, ubuntu-latest, 3.7, py37)	2m 55s
test (py3.8, ubuntu-latest, 3.8, py38)	2m 37s
test (py3.9, ubuntu-latest, 3.9, py39)	2m 46s
test (py3.10, ubuntu-latest, 3.10, py310)	2m 38s
test (py3.10 with gmpy, ubuntu-latest, 3.10, gmpypy310)	2m 38s
test (py3.10 with gmpy2, ubuntu-latest, 3.10, gmpy2py310)	2m 35s
test (py3.11, ubuntu-latest, 3.11, py311)	2m 35s
test (py3.12, ubuntu-latest, 3.12.0-beta.1, py312)	2m 59s
test (pypy, ubuntu-latest, pypy-2.7, pypy)	4m 6s
test (pypy3, ubuntu-latest, pypy-3.7, pypy3)	4m 40s
test (py2.7 with instrumental, ubuntu-20.04, 2.7, instrumental)	3m 17s
test (code checks, ubuntu-latest, 3.9, codechecks)	26s
test (mutation testing, ubuntu-latest, 3.11, true)	3m 21s
Indicate completion to coveralls.io	0s
Prepare job files for the mutation runners	0s
Execute mutation testing	0s
Combine mutation testing results	0s
	1h 4m 43s

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