Pex

This release fixes both PEX creation and `--venv` creation to handle
distributions that contain scripts with non-ascii characters in them
when running in environments with a default encoding that does not
contain those characters under PyPy3, Python 3.5 and Python 3.6.

* Fix non-ascii script shebang re-writing. (1480)

This is another hotfix of the 2.1.48 release's `--layout` feature that
fixes identification of `--layout zipapp` PEXes that have had their
execute mode bit turned off. A notable example is the Pex PEX when
downloaded from <https://github.com/pex-tool/pex/releases>.

* Fix zipapp layout identification. (1448)

This is a hotfix release that fixes the new `--layout {zipapp,packed}`
modes for PEX files with no user code & just third party dependencies
when executed against a `$PEX_ROOT` where similar PEXes built with the
old `--not-zip-safe` option were run in the past.

* Avoid re-using old ~/.pex/code/ caches. (1444)

This releases introduces the `--layout` flag for selecting amongst the
traditional zipapp layout as a single PEX zip file and two new directory
tree based formats that may be useful for more sophisticated deployment
scenarios.

The `--unzip` / `PEX_UNZIP` toggles for PEX runtime execution are now
the default and deprecated as explicit options as a result. You can
still select the venv runtime execution mode via the `--venv` /
`PEX_VENV` toggles though.

* Remove zipapp execution mode & introduce `--layout`. (1438)

This is a hotfix release that fixes a regression for `--venv` mode PEXes
introduced in 1410. These PEXes were not creating new venvs when the
PEX was unconstrained and executed with any other interpreter than the
interpreter the venv was first created with.

* Fix `--venv` mode venv dir hash. (1428)
* Clarify PEX_PYTHON & PEX_PYTHON_PATH interaction. (1427)

This release improves PEX file build reproducibility and requirement
parsing of environment markers in Pip's proprietary URL format.

Also, the `-c` / `--script` / `--console-script` argument now supports
non-Python distribution scripts.

Finally, new contributor blag improved the README.

* Fix Pip proprietary URL env marker handling. (1417)
* Un-reify installed wheel script shebangs. (1410)
* Support deterministic repository extract tool. (1411)
* Improve examples and add example subsection titles (1409)
* support any scripts specified in `setup(scripts=...)`
from setup.py. (1381)