**Summary**
Initial attempts to incrementally refactor `partition_email()` into
shape to allow pluggable partitioning quickly became too complex for
ready code-review. Prepare separate rewritten module and tests and swap
them out whole.
**Additional Context**
- Uses the modern stdlib `email` module to reliably accomplish several
manual decoding steps in the legacy code.
- Remove obsolete email-specific element-types which were replaced 18
months or so ago with email-specific metadata fields for things like Cc:
addresses, subject, etc.
- Remove accepting an email as `text: str` because MIME-email is
inherently a binary format which can and often does contain multiple and
contradictory character-encodings.
- Remove `encoding` parameters as it is now unused. An email file is not
a text file and as such does not have a single overall encoding.
Character encoding is specified individually for each MIME-part within
the message and often varies from one part to another in the same
message.
- Remove the need for a caller to specify `attachment_partitioner`.
There is only one reasonable choice for this which is
`auto.partition()`, consistent with the same interface and operation in
`partition_msg()`.
- Fixes#3671 along the way by silently skipping attachments with a
file-type for which there is no partitioner.
- Substantially extend the test-suite to cover multiple
transport-encoding/charset combinations.
---------
Co-authored-by: ryannikolaidis <1208590+ryannikolaidis@users.noreply.github.com>
Co-authored-by: scanny <scanny@users.noreply.github.com>
