Update code to run under Python3 + Conda packaging

• Updated to work with Python 3 (resources used: 2to3, caniusepython3, pylint, flak8, mypy)
• Added some unit tests to ensure basic functionality of julday
• Updated list of platform specific dependencies to be installed when installing julday in dev mode (see setup.py)
• Installed and tested julday against Python3.[5,6,7,8] using tox
• Formatted Python code to conform to the PEP8 style guide
• Created conda packages for "julday" that can run on Python3.[5,6,7,8]
• Updated .gitlab-ci.yml to run a linter and unit tests for Python3.[5,6,7,8] in GitLab CI pipeline