db: move URL_SEP constant from db to top level kallithea module URL_SEP is used in several places - having it in the db module is too high level. __init__ might not be the best place for this, but it does no harm to the dependencies graph to place constants there.

pytest >= 4.6.6, < 5.5
pytest-sugar >= 0.9.2, < 0.10
pytest-benchmark >= 3.2.2, < 3.3
pytest-localserver >= 0.5.0, < 0.6
mock >= 3.0.0, < 4.1
Sphinx >= 1.8.0, < 3.1
WebTest >= 2.0.6, < 2.1
isort == 5.1.2
pyflakes == 2.2.0