Manypex

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* *API change*: Decouple translation from package iteration. This removes
the Obtainer construct entirely, which likely means if you're using PEX as
a library, you will need to change your code if you were doing anything
nontrivial. This adds a couple new options to ``resolve`` but simplifies
the story around how to cache packages.
`RB 785 <https://rbcommons.com/s/twitter/r/785/>`_

* Refactor http handling in pex to allow for alternate http implementations. Adds support
for `requests <https://github.com/kennethreitz/requests>`_,
improving both performance and security. For more information, read the commit notes at
`91c7f32 <https://github.com/pantsbuild/pex/commit/91c7f324085c18af714d35947b603a5f60aeb682>`_.
`RB 778 <https://rbcommons.com/s/twitter/r/778/>`_

* Improvements to API documentation throughout.

* Renamed ``Tracer`` to ``TraceLogger`` to prevent nondeterministic isort ordering.

* Refactor tox.ini to increase the number of environment combinations and improve coverage.

* Adds HTTP retry support for the RequestsContext.
`RB 1303 <https://rbcommons.com/s/twitter/r/1303/>`_

* Make pex --version correct.
`Issue 19 <https://github.com/pantsbuild/pex/issues/19>`_

* Bug fix: Fix over-aggressive sys.modules scrubbing for namespace packages. Under
certain circumstances, namespace packages in site-packages could conflict with packages
within a PEX, causing them to fail importing.
`RB 1378 <https://rbcommons.com/s/twitter/r/1378/>`_

* Bug fix: Replace uses of ``os.unsetenv(...)`` with ``del os.environ[...]``
`Pull Request 11 <https://github.com/pantsbuild/pex/pull/11>`_

* Bug fix: Scrub sys.path and sys.modules based upon both supplied path and
realpath of files and directories. Newer versions of virtualenv on Linux symlink site-packages
which caused those packages to not be removed from sys.path correctly.
`Issue 21 <https://github.com/pantsbuild/pex/issues/21>`_

* Bug fix: The pex -s option was not correctly pulling in transitive dependencies.
`Issue 22 <https://github.com/pantsbuild/pex/issues/22>`_

* Bug fix: Adds ``content`` method to HTTP contexts that does HTML content decoding, fixing
an encoding issue only experienced when using Python 3.
`Issue 10 <https://github.com/pantsbuild/pex/issues/10>`_

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* Rename ``twitter.common.python`` to ``pex`` and split out from the
`twitter/commons <http://github.com/twitter/commons>`_ repo.

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* Change the interpretation of ``-i`` (and of PyPIFetcher's pypi_base)
to match pip's ``-i``. This is useful for compatibility with devpi.

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* Ensures that .egg/.whl distributions on disk have their mtime updated
even though we no longer overwrite them. This gives them a new time
lease against their ttl.

Without this change, once a distribution aged past the ttl it would
never be used again, and builds would re-create the same distributions
in tmpdirs over and over again.

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* Fixes an issue where SourceTranslator would overwrite .egg/.whl
distributions already on disk. Instead it should always check to see if
a copy already exists and reuse if there.

This ordinarily should not be a problem but the zipimporter caches
metadata by filename instead of stat/sha, so if the underlying contents
changed a runtime error would be thrown due to seemingly corrupt zip file
offsets. `RB 684 <https://rbcommons.com/s/twitter/r/684/>`_

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* Adds ``-i/--index`` option to the pex tool.