Add markers for tests requiring network connection

All Linux distributions (and I guess some CIs as well) block any network connections to the isolated environments. Would it be possible to add pytest [marker](https://docs.pytest.org/en/latest/example/markers.html) marking all test cases requiring network access (currently, it is only ``test_get_changelog_url``)? Thank you.

[Possible patch for the current tarball](https://github.com/dephell/dephell_changelogs/files/4277912/add_network_markers.patch.txt)