Switch vendoring from `pip install ...` to Pex install. (#2306) #2368

Job	Run time
Check GitHub Organization	1s
tox -e format-check,lint-check,typecheck,vendor-check,package	4m 34s
tox -e py312-pip23_3_1	18m 7s
tox -e py312-pip23_3_1-integration	1h 11m 29s
./dtox.sh -e py27-pip20	11m 38s
./dtox.sh -e py35-pip20	13m 24s
./dtox.sh -e py311-pip20	14m 38s
./dtox.sh -e py311-pip22_3_1	15m 13s
./dtox.sh -e py311-pip23_1_2	14m 43s
./dtox.sh -e py312-pip23_3_1	14m 14s
./dtox.sh -e pypy310-pip20	25m 22s
./dtox.sh -e pypy310-pip22_3_1	24m 36s
./dtox.sh -e pypy310-pip23_1_2	24m 59s
./dtox.sh -e py27-pip20-integration	19m 36s
./dtox.sh -e py37-pip22_3_1-integration	42m 38s
./dtox.sh -e py37-pip23_1_2-integration	41m 25s
./dtox.sh -e py311-pip20-integration	25m 54s
./dtox.sh -e py311-pip22_3_1-integration	27m 26s
./dtox.sh -e py311-pip23_1_2-integration	29m 42s
./dtox.sh -e py312-pip23_3_1-integration	35m 42s
./dtox.sh -e pypy310-pip20-integration	45m 33s
./dtox.sh -e pypy310-pip22_3_1-integration	54m 40s
./dtox.sh -e pypy310-pip23_1_2-integration	53m 21s
Gather Final Status	1s
	10h 28m 56s

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