hg: handle Mercurial RepoError correctly in is_valid_repo_uri RepoError would be leaked by is_valid_repo_uri. Now, when for example validating an ssh URL without having the ssh client binary, it will be shown/logged as: remote: /bin/sh: ssh: command not found 2020-03-17 17:28:53.907 WARNI [kallithea.model.validators] validation of clone URL 'ssh://no-ssh.com/' failed: Mercurial RepoError: no suitable response from remote hg and shown in the UI as 'Invalid repository URL'.

# requirements.txt file for use as "pip install -r requirements.txt" as a
# readthedocs compatible alternative to "pip install -e ." which is a working
# alternative to "setup.py develop" which doesn't work with Mercurial 3.7
.