Zest.releaser

--------------------

- Bug 381: In ``prerelease``, check with ``pep440`` if the version is canonical.
Added ``pep440`` to the ``recommended`` extra, not to the core dependencies:
``zest.releaser`` can also be used for non-Python projects.
[maurits]

--------------------

- Add ``--headless`` option to ``longtest``.

--------------------

Big cleanup to ease future development:

- Removed support for Subversion (``svn``), Bazaar (``bzr``), Mercurial (``hg``).

- Removed support for Python 2 and 3.5.

- Added support for Python 3.9 and 3.10.

- Tested with Python 3.6-3.10 plus PyPy3.

- Switched from Travis to GitHub Actions.

- Simplified running commands by using ``subprocess.run``.


.. Note: for older changes see ``doc/sources/changelog.rst``.