Errata-tool

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* Build system: fix syntax error in Makefile

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* Build system: avoid stray files in tarball during sdist build

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* Add RHSA support (new ``security_impact`` kwarg when creating advisories)

* Add a new ``reloadBuilds()`` method to the ``Erratum`` class to reload an
advisory's product listings.

* Discover the Kerberos username in the ``ErrataConnector`` class.

* Add a new ``.content_types`` attribute to the ``Erratum`` class to discover
if an advisory is an RPM or Docker advisory.

* Add a new ``metadataCdnRepos()`` method to the ``Erratum`` class to set or
get the CDN repositories for a container advisory.

* Add a new ``externalTests()`` method to the ``Erratum`` class to discover the
state of RPMDiff tests.

* Add a new ``advisories()`` method to the ``Release`` class to discover all
advisories for a release.

* Add a new ``push()`` method to the ``Erratum`` class to push content to the
stage or live CDN.

* Add basic ``errata-tool`` CLI.

* Several documentation fixes

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* Add new Product, User, and Release classes

* Disable mutual auth for all HTTPS requests

* Code linting cleanup

* Include tests and license in source distribution

* Remove rpmdiff support (rpmdiff is now decoupled from ET)

* More examples in README

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* Fix traceback in ``ProductList`` if a release has no versions associated.

* Fix ability to change an existing advisory to be text-only or non-text-only.

* Basic PDC support: Gracefully handle PDC prefixes for advisory types.

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* New project URL: https://github.com/red-hat-storage/errata-tool

* Avoid re-adding the RHSA severity prefix to an advisory's synopsis when
making unrelated updates.