Errata-tool

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* Respect the batch_id for an ``Erratum`` attribute when calculating
``.publish_date`` and the class ``str`` representation. The
``errata-tool advisory get <advisory_id>`` command now shows the batch ID if
the advisory is part of a batch update.

* Fix the ability to set a new severity value on existing advisories with
``Erratum.update(security_impact=<value>)``.

* CLI: code optimization make it easier to add new sub-commands.

* RPM packaging is Python 3-only on Fedora and RHEL 8.

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* Lazy-load full Erratum information within the ``Build`` class.
Prior to this change, when users queried a build, python-errata-tool would
immediately load a lot of information about every advisory for that build.
With this change, the ``Build`` class only loads the full advisories when
users access the ``.released_errata`` property.

Users can also access the advisory ID numbers without loading all of the
information about each advisory. This change adds two new properties to the
``Build`` class: ``.released_errata_id`` and ``.all_errata_ids``.

* Get paginated data in the ``Erratum.externalTests()`` method. Prior to this
change, ``externalTests()`` would only return the first few test results.
The ``externalTests()`` method now returns the entire list of test results.

* CLI: add a ``--status OPEN`` alias to the
``errata-tool release list-advisories`` command. This allows users to list
all the in-progress advisories for a release.

* Erratum: fix a crash when the Errata Tool returns ``None`` for a
SHIPPED_LIVE advisory's ``actual_ship_date``.

* Clean up API documentation

* Test suite improvements

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This release deletes all references to PDC (rhbz1692965). In particular this
removes:

* The ``.is_pdc`` attribute from ``Release`` class

* The ``.supports_pdc`` attribute from ``Product`` class

* Support for reading or updating PDC-type advisories (``Erratum`` class)

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* Add a new ``.get_erratum_data()`` method to the ``Erratum`` class to
query the server's JSON for an advisory. Use this when debugging
interactions with the Errata Tool or when passing this data on to other
non-Python tools.

* Add a new ``.publish_date_override`` attribute to the ``Erratum`` class to
get the overridden date for an advisory.

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* Make the ``.package_owner_email`` attribute to the ``Erratum`` class
correspond to the Errata Tool's "package owner" for an advisory. Prior to
this change, ``.package_owner_email`` was the advisory reporter, not the
package owner.

* Add a new ``.reporter`` attribute to the ``Erratum`` class to
get the reporter email address for an advisory.

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* Add a new ``.manager_id`` attribute to the ``Erratum`` class to
get the manager for an advisory. You can now optionally set the manager for
a new advisory using the manager ID number instead of an email address. This
makes it easier to clone advisories.

* Fix the ``__str__`` method for the ``User`` class to print a real value
instead of crashing.