Errata-tool

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* The ``render()`` method on the ``ProductVersion`` class now displays the
``allow_rhn_debuginfo`` value for each Product Version. The ``dump-config``
command now prints this configuration setting.

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* Erratum instances have a new ``.jira_issues`` attribute. This is a list of
the jira ticket IDs attached to the advisory.

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* CLI: do not print duplicate CDN repositories in ``dump-config`` sub-command.

* README: Fix code examples for ``metadataCdnRepos()`` and
``textOnlyRepos()``.

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* CLI: add a new ``dump-config`` sub-command. This makes it easier to dump a
product's existing Errata Tool configuration into a format that Ansible can
read later. This makes it easier for release engineers to transition
products to use errata-tool-ansible.

* Add a new ``releases()`` method to the ``Product`` class. This returns a
list of releases associated with a product.

* Add a new ``render()`` method to the ``Release`` class. This makes it easier
to obtain the data from this class in a structured way.

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* Replace requests-kerberos dependency with requests-gssapi. requests-kerberos
is not well-maintained upstream and Fedora is dropping it entirely.
requests-gssapi is an up-to-date replacement.

* Remove the ``._username`` property from the ``ErrataConnector`` class. The
purpose of this property was to discover the Kerberos username. This is
unnecessary and difficult to support with the switch to
python-requests-gssapi.

* Add a new ``product_versions()`` method to the ``Product`` class. This
returns a list of product versions associated with a product.

* Update ``Product`` and ``ProductVersion`` classes to use the Errata Tool's
newer v1 API endpoints. This provides more data fields on these classes.

* Add a new ``render()`` method to the ``Product``, ``ProductVersion``, and
``Variant`` classes. This makes it easier to obtain the data from these
classes in a structured way.

* Make the unit test suite pass on EPEL 7's pytest 2.7.0.

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* Add a new ``CdnRepo`` class to represent Errata Tool CDN repository records.
Add a new ``cdn_repos()`` method to the ``Variant`` class to find a list of
cdn repos associated with a product variant.

* Add a new ``variants()`` method to the ``ProductVersion`` class to find
the list of variants associated with a product version.

* Add a new ``params`` kwarg to the ``ErrataConnector._get()`` method. This
properly encodes values for query strings in GET urls.

* Handle "+" characters in ``Release`` names.

* Fix the ability to modify owner emails and manager emails on existing
advisories with ``Erratum.update(owner_email=<value>)`` or
``Erratum.update(manager_email=<value>)``.

* Improve project CI with GitHub Actions and codecov.io.

* Fix warnings in RPM packaging