Flask-bower

- drop python 2.6 support / six dependency
- switch build infrastructure to pytest and GitHub Actions
- switch to `.bower.json` as per recommendation - thanks `ivelin`_
- Fix ModuleNotFoundError - thanks `jeffhung`_

.. _ivelin: https://github.com/lobeck/flask-bower/pull/9
.. _jeffhung: https://github.com/lobeck/flask-bower/pull/10

- Support absolute paths in BOWER_COMPONENT_ROOT - thanks `jesseops`_

.. _jesseops: https://github.com/lobeck/flask-bower/pull/6

- remove BuildError handling since flask is already taking care (glitch in the flask documentation)

- add conditional switch on ``send_file`` to ensure 304 responses

- flasks default ``url_for`` is now supported for bower assets - requires Flask >= 0.9
- added ``BOWER_KEEP_DEPRECATED`` option
- added ``BOWER_REPLACE_URL_FOR`` option
- ``bower_url_for`` is now deprecated

- ``bower.json`` is now optional since it may be not available if files are pulled from a random source which is not supporting bower