Pex

This is another hotfix release to fix a regression in Pex
`--sources-directory` handling of relative paths.

* Support relative paths in `Chroot.symlink`. (1194)

This is a hotfix release that fixes requirement parsing when there is a
local file in the CWD with the same name as the project name of a remote
requirement to be resolved.

* Requirement parsing handles local non-dist files. (1190)

This release brings support for a `--venv` execution mode to complement
`--unzip` and standard unadorned PEX zip file execution modes. The
`--venv` execution mode will first install the PEX file into a virtual
environment under `${PEX_ROOT}/venvs` and then re-execute itself from
there. This mode of execution allows you to ship your PEXed application
as a single zipfile that automatically installs itself in a venv and
runs from there to eliminate all PEX startup overhead on subsequent runs
and work like a "normal" application.

There is also support for a new resolution mode when building PEX files
that allows you to use the results of a previous resolve by specifying
it as a `-pex-repository` to resolve from. If you have many applications
sharing a requirements.txt / constraints.txt, this can drastically speed
up resolves.

* Improve PEX repository error for local projects. (1184)
* Use symlinks to add dists in the Pex CLI. (1185)
* Suppress `pip debug` warning. (1183)
* Support resolving from a PEX file repository. (1182)
* PEXEnvironment for a DistributionTarget. (1178)
* Fix plumbing of 2020-resolver to Pip. (1180)
* Platform can report supported_tags. (1177)
* Record original requirements in PEX-INFO. (1171)
* Tighten requirements parsing. (1170)
* Type BuildAndInstallRequest. (1169)
* Type AtomicDirectory. (1168)
* Type SpawnedJob. (1167)
* Refresh and type OrderedSet. (1166)
* PEXEnvironment recursive runtime resolve. (1165)
* Add support for `-r` / `--constraints` URL to the CLI. (1163)
* Surface Pip dependency conflict information. (1162)
* Add support for parsing extras and specifiers. (1161)
* Support project_name_and_version metadata. (1160)
* docs: fix simple typo, original -> original (1156)
* Support a `--venv` mode similar to `--unzip` mode. (1153)
* Remove redundant dep edge label info. (1152)
* Remove our reliance on packaging's LegacyVersion. (1151)
* Implement PEX_INTERPRETER special mode support. (1149)
* Fix PexInfo.copy. (1148)

This release upgrades Pip to 20.3.3 + a patch to fix Pex resolves using
the `pip-legacy-resolver` and `--constraints`. The Pex package is also
fixed to install for Python 3.9.1+.

* Upgrade to a patched Pip 20.3.3. (1143)
* Fix python requirement to include full 3.9 series. (1142)

This release upgrades Pex to the latest Pip which includes support for
the new 2020-resolver (see:
<https://pip.pypa.io/en/stable/user_guide/#resolver-changes-2020>) as
well as support for macOS BigSur. Although this release defaults to the
legacy resolver behavior, the next release will deprecate the legacy
resolver and support for the legacy resolver will later be removed to
allow continuing Pip upgrades going forward. To switch to the new
resolver, use: `--resolver-version pip-2020-resolver`.

* Upgrade Pex to Pip 20.3.1. (1133)

This release fixes a deadlock that could be experienced when building
PEX files in highly concurrent environments in addition to fixing
`pex --help-variables` output.

A new suite of PEX tools is now available in Pex itself and any PEXes
built with the new `--include-tools` option. Use
`PEX_TOOLS=1 pex --help` to find out more about the available tools and
their usage.

Finally, the long deprecated exposure of the Pex APIs through `_pex` has
been removed. To use the Pex APIs you must include pex as a dependency
in your PEX file.

* Add a dependency graph tool. (1132)
* Add a venv tool. (1128)
* Remove long deprecated support for _pex module. (1135)
* Add an interpreter tool. (1131)
* Escape venvs unless PEX_INHERIT_PATH is requested. (1130)
* Improve `PythonInterpreter` venv support. (1129)
* Add support for PEX runtime tools & an info tool. (1127)
* Exclusive atomic_directory always unlocks. (1126)
* Fix `PythonInterpreter` binary normalization. (1125)
* Add a `requires_dists` function. (1122)
* Add an `is_exe` helper. (1123)
* Fix req parsing for local archives & projects. (1121)
* Improve PEXEnvironment constructor ergonomics. (1120)
* Fix `safe_open` for single element relative paths.
(1118)
* Add URLFetcher IT. (1116)
* Implement full featured requirement parsing. (1114)
* Fix `--help-variables` docs. (1113)
* Switch from optparse to argparse. (1083)