Setup.py: update classifiers, add python_requires and long_description

[hugovk]

Some handy updates to setup.py:

• Update Trove classifiers so the supported versions are clear on PyPI https://pypi.org/project/gpxpy/
• Use setuptools rather than distutils for:
  • Add python_requires, so pip knows which version of this package to install for people's installed Python version
  • Add the README.md contents as long_description so https://pypi.org/project/gpxpy/ will show the README instead of "The author of this package has not provided a project description"

For these, make sure you have pip install -U setuptools twine and use twine for upload.

See:

• https://dustingram.com/articles/2018/03/16/markdown-descriptions-on-pypi
• https://github.com/di/markdown-description-example

[coveralls]

Coverage increased (+0.1%) to 85.453% when pulling 68e2866 on hugovk:update-setup into 1f344e9 on tkrajina:master.

[tkrajina]

Thanks.