Changes:
* add shield badges for PyPI and GitHub releases
* fix rendering of code blocks in ``README.rst``
* add missing classifiers and python requirements to ``setup.py`` to allow validators to detect appropriate versions
* add python 3.9, 3.10 and 3.11 to the supported versions in ``setup.py`` and validate them in GitHub CI
* drop Travis CI configuration in favor of GitHub CI
* fix ``test_noaa`` with the target THREDDS server responding differently than originally tested