Add release notes for 0.19.0 release #276

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 49s
test (py2.7, ubuntu-20.04, 2.7, py27)	2m 43s
test (py2.7 with old gmpy, ubuntu-20.04, 2.7, py27_old_gmpy)	58s
test (py2.7 with old gmpy2, ubuntu-20.04, 2.7, py27_old_gmpy2)	3m 12s
test (py2.7 with old six, ubuntu-20.04, 2.7, py27_old_six)	2m 49s
test (py2.7 with gmpy, ubuntu-20.04, 2.7, gmpypy27)	2m 42s
test (py2.7 with gmpy2, ubuntu-20.04, 2.7, gmpy2py27)	2m 48s
test (py3.5, ubuntu-20.04, 3.5, py35)	2m 43s
test (py3.6, ubuntu-20.04, 3.6, py36)	2m 44s
test (py3.7, ubuntu-latest, 3.7, py37)	2m 42s
test (py3.8, ubuntu-latest, 3.8, py38)	3m 6s
test (py3.9, ubuntu-latest, 3.9, py39)	2m 51s
test (py3.10, ubuntu-latest, 3.10, py310)	2m 44s
test (py3.10 with gmpy, ubuntu-latest, 3.10, gmpypy310)	2m 54s
test (py3.10 with gmpy2, ubuntu-latest, 3.10, gmpy2py310)	2m 42s
test (py3.11, ubuntu-latest, 3.11, py311)	3m 0s
test (py3.12, ubuntu-latest, 3.12.0-beta.1, py312)	3m 14s
test (pypy, ubuntu-latest, pypy-2.7, pypy)	2m 58s
test (pypy3, ubuntu-latest, pypy-3.7, pypy3)	1m 22s
test (py2.7 with instrumental, ubuntu-20.04, 2.7, instrumental)	4m 13s
test (code checks, ubuntu-latest, 3.9, codechecks)	23s
test (mutation testing, ubuntu-latest, 3.11, true)	1m 5s
Indicate completion to coveralls.io	1s
Prepare job files for the mutation runners	1s
Execute mutation testing	1s
Combine mutation testing results	1s
	58m 56s

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