Travis CI Linux Environments are unable to clone mercurial repositories from bitbucket.org, since they’ve disabled support for TLS 1.0 and 1.1 and the mercurial clients in the Linux Environment are not up to date enough.
https://bitbucket.org/blog/deprecating-tlsv1-tlsv1-1-2018-12-01
https://www.mercurial-scm.org/wiki/SecureConnections
warning: connecting to bitbucket.org using legacy security technology (TLS 1.0); see https://mercurial-scm.org/wiki/SecureConnections for more info
(using CA certificates from /etc/ssl/certs/ca-certificates.crt; if you see this message, your Mercurial install is not properly configured; see https://mercurial-scm.org/wiki/SecureConnections for how to configure Mercurial to avoid this message)
abort: error: _ssl.c:510: error:1408F10B:SSL routines:SSL3_GET_RECORD:wrong version number
Link to an actual failure: https://travis-ci.org/pypa/pip/jobs/462231808