Fixed all dead URLs. Removed travis related files. Re-ordered dependencies.

*Added Python 3.9 classifier to setup.py and removed those for <3.6.

Signed-off-by: Daniel Scheffler <danschef@gfz-potsdam.de>
3 jobs for feature/add_urlchecker in 1 minute and 11 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Test
passed test_py_tools_ds #52540

00:00:13

68.0%
passed test_styles #52541

00:00:08

passed test_urls #52542

00:00:49