Pex

* Fix `iter_compatible_interpreters` with `path`. (1110)
* Fix `Requires-Python` environment marker mapping. (1105)
* Fix spurious `InstalledDistribution` env markers. (1104)
* Deprecate `-R`/`--resources-directory`. (1103)
* Fix ResourceWarning for unclosed `/dev/null`. (1102)
* Fix runtime vendoring bytecode compilation races. (1099)

This release improves interpreter discovery to prefer more recent patch
versions, e.g. preferring Python 3.6.10 over 3.6.8.

We recently regained access to the docsite, and
<https://docs.pex-tool.org/> is now up-to-date.

* Prefer more recent patch versions in interpreter discovery. (1088)
* Fix `--pex-python` when it's the same as the current interpreter.
(1087)
* Fix `dir_hash` vs. bytecode compilation races. (1080)
* Fix readthedocs doc generation. (1081)

This release adds the `--python-path` option, which allows controlling
the interpreter search paths when building a PEX.

The release also removes `--use-first-matching-interpreter`, which was a
misfeature. If you want to use fewer interpreters when building a PEX,
use more precise values for `--interpreter-constraint` and/or
`--python-path`, or use `--python` or `--platform`.

* Add `--python-path` to change interpreter search paths when building
a PEX. (1077)
* Remove `--use-first-matching-interpreter` misfeature. (1076)
* Encapsulate `--inherit-path` handling. (1072)

This release brings official support for Python 3.9 and adds a new
`--tmpdir` option to explicitly control the TMPDIR used by Pex and its
subprocesses. The latter is useful when building PEXes in
space-constrained environments in the face of large distributions.

The release also fixes `--cert` and `--client-cert` so that they work
with PEP-518 builds in addition to fixing bytecode compilation races in
highly parallel environments.

* Add a `--tmpdir` option to the Pex CLI. (1068)
* Honor `sys.executable` unless macOS Framework. (1065)
* Add Python 3.9 support. (1064)
* Fix handling of `--cert` and `--client-cert`. (1063)
* Add atomic_directory exclusive mode. (1062)
* Fix `--cert` for PEP-518 builds. (1060)

This release fixes a bug in `--resolve-local-platforms` handling that
made it unusable in 2.1.16 (1043) as well as fixing a long-standing
file handle leak (1050) and a bug when running under macOS framework
builds of Python (1009).

* Fix `--unzip` performance regression. (1056)
* Fix resource leak in Pex self-isolation. (1052)
* Fix use of `iter_compatible_interpreters`. (1048)
* Do not rely on `sys.executable` being accurate. (1049)
* slightly demystify the relationship between platforms and
interpreters in the library API and CLI (1047)
* Path filter for PythonInterpreter.iter_candidates. (1046)
* Add type hints to `util.py` and `tracer.py`
* Add type hints to variables.py and platforms.py (1042)
* Add type hints to the remaining tests (1040)
* Add type hints to most tests (1036)
* Use MyPy via type comments (1032)

This release fixes a bug in `sys.path` scrubbing / hermeticity (1025)
and a bug in the `-D / --sources-directory` and
`-R / --resources-directory` options whereby PEP-420 implicit
(namespace) packages were not respected (1021).

* Improve UnsatisfiableInterpreterConstraintsError. (1028)
* Scrub direct `sys.path` manipulations by .pth files. (1026)
* PEX zips now contain directory entries. (1022)
* Fix UnsatisfiableInterpreterConstraintsError. (1024)