Switch to setuptools for the Pex build. (#2599) #3208

Job	Run time
Check GitHub Organization	1s
tox -e format-check,lint-check,typecheck,enum-check,vendor-check,package,docs	10m 54s
tox -e py313-pip24_3_1	13m 46s
tox -e py313-pip24_3_1-integration --shard 1/2	42m 50s
./dtox.sh -e py27-pip20	10m 56s
tox -e py313-pip24_3_1-integration --shard 2/2	26m 41s
./dtox.sh -e py38-pip22_3_1	18m 21s
./dtox.sh -e py311-pip23_3_2	18m 27s
./dtox.sh -e py313-pip24_3_1	14m 41s
./dtox.sh -e py314-pip24_3_1	15m 41s
./dtox.sh -e pypy310-pip24_3_1	27m 52s
./dtox.sh -e py27-pip20-integration	28m 53s
./dtox.sh -e py38-pip22_3_1-integration --shard 1/2	33m 11s
./dtox.sh -e py38-pip22_3_1-integration --shard 2/2	28m 37s
./dtox.sh -e py311-pip23_3_2-integration --shard 1/2	31m 9s
./dtox.sh -e py311-pip23_3_2-integration --shard 2/2	28m 57s
./dtox.sh -e py313-pip24_3_1-integration --shard 1/2	26m 30s
./dtox.sh -e py313-pip24_3_1-integration --shard 2/2	27m 54s
./dtox.sh -e py314-pip24_3_1-integration --shard 1/2	26m 51s
./dtox.sh -e py314-pip24_3_1-integration --shard 2/2	28m 29s
./dtox.sh -e pypy310-pip24_3_1-integration --shard 1/2	46m 22s
./dtox.sh -e pypy310-pip24_3_1-integration --shard 2/2	50m 0s
Gather Final Status	1s
	9h 17m 4s

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