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* added experimental `_local_to_host_pid` method to Shell API.

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* added support for mypy type hint discovery as specified in PEP 561
* added missing type annotations

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* updated :code:`Shell` construction to raise a :code:`ContainerFileNotFound`
if a given source does not exist.
* added :code:`access` method to `FileSystem` for testing whether user inside
shell has permission to perform operations on a given file.

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* updated :code:`TimeoutExpired` and :code:`CalledProcessError` exceptions to
inherit from their counterparts in the Python standard library
* added :code:`encoding` parameter to :code:`Popen` to allow users to specify
which encoding, if any, should be used to decode the output stream from a
process.
* added :code:`volumes` parameter to :code:`provision` method in
:code:`DockerDaemon`, allowing users to specify volume mounts.

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* added `persist` method to `Container`
* added `pid` property `Container`
* added `_info` property to `Container`
* added `find` method to `FileSystem`
* added `listdir` method to `FileSystem`
* added `mkdir` method to `FileSystem`
* added `rmdir` method to `FileSystem`
* added ability to specify an environment via `environment` argument of
`shell` method in `Container`
* added ability to construct a shell using sources via `sources` argument
of `shell` method in `Container`
* optimised implementation of `environ` method in `Shell` by removing the
need to interact with the underlying container

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* added ip_address property to Container