update CI to make 3.7 continue to work #305

Job	Run time
test (py2.6, ubuntu-latest, centos:6, 2.6)	2m 10s
test (py3.10 with ossl3.0, ubuntu-latest, ubuntu:22.04, 3.10, py310)	2m 52s
test (py2.7, ubuntu-20.04, 2.7, py27)	2m 51s
test (py2.7 with old gmpy, ubuntu-20.04, 2.7, py27_old_gmpy)	1m 0s
test (py2.7 with old gmpy2, ubuntu-20.04, 2.7, py27_old_gmpy2)	2m 50s
test (py2.7 with old six, ubuntu-20.04, 2.7, py27_old_six)	2m 55s
test (py2.7 with gmpy, ubuntu-20.04, 2.7, gmpypy27)	2m 56s
test (py2.7 with gmpy2, ubuntu-20.04, 2.7, gmpy2py27)	2m 46s
test (py3.6, ubuntu-20.04, 3.6, py36)	2m 53s
test (py3.7, ubuntu-22.04, 3.7, py37)	2m 50s
test (py3.8, ubuntu-latest, 3.8, py38)	2m 48s
test (py3.9, ubuntu-latest, 3.9, py39)	2m 41s
test (py3.10, ubuntu-latest, 3.10, py310)	2m 48s
test (py3.10 with gmpy, ubuntu-latest, 3.10, gmpypy310)	2m 33s
test (py3.10 with gmpy2, ubuntu-latest, 3.10, gmpy2py310)	2m 39s
test (py3.11, ubuntu-latest, 3.11, py311)	2m 45s
test (py3.12, ubuntu-latest, 3.12, py312)	2m 43s
test (py3.13, ubuntu-latest, 3.13, py313)	2m 24s
test (pypy, ubuntu-latest, pypy-2.7, pypy)	2m 40s
test (pypy3, ubuntu-22.04, pypy-3.7, pypy3)	4m 47s
test (py2.7 with instrumental, ubuntu-20.04, 2.7, instrumental)	4m 1s
test (code checks, ubuntu-latest, 3.9, codechecks)	22s
test (mutation testing, ubuntu-latest, 3.11, true)	1m 3s
Indicate completion to coveralls.io	0s
Prepare job files for the mutation runners	0s
Execute mutation testing	0s
Combine mutation testing results	0s
	1h 0m 17s

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