chore(deps): update dependency pex to v2.20.3

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
pex (changelog)	2.20.2 -> 2.20.3

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Release Notes

pex-tool/pex (pex)

v2.20.3

Compare Source

This release fixes both PEX building and lock creation via
pex3 lock {create,sync} to be reproducible in more cases. Previously,
if a requirement only available in source form (an sdist, a local
project or a VCS requirement) had a build that was not reproducible due
to either file timestamps (where the SOURCE_DATE_EPOCH standard was
respected) or random iteration order (e.g.: the setup.py used sets in
certain in-opportune ways), Pex's outputs would mirror the problematic
requirement's non-reproducibility. Now Pex plumbs a fixed
SOURCE_DATE_EPOCH and PYTHONHASHSEED to all places sources are
built.

• Plumb reproducible build env vars more thoroughly. (#​2554)

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