It is not completely working yet.

This fixes a problem where the tests for fetching a nonexistent ref
prompt like "Enter passphrase for key '/home/USERNAME/.ssh/id_rsa':"
and block, if the repository on the machine where the tests are
being run has the remote set up using an SSH URL.

This passes through all environment variables whose names start
with SSH_, even though it should be enough to pass SSH_AGENT_PID
and SSH_AUTH_SOCK through, at least for this particular issue.

Other environments would still be run even after mypy has failed,
but to avoid having tox runs be unnecessarily inconsistent with the
mypy step in the pythonpackage.yml CI workflow, and also because
GitPython is not currently expected to pass mypy checks, this keeps
mypy errors from causing the whole tox run to be reported as
failed.

The main use of this, similar to the step at the end of
pythonpackage.yml, is to find errors produced by building.

However, actual documentation *is* built, and unlike other tox
environments, running this one actually writes outside the .tox/
directory, creating the documentation in the usual target
location. For that reason, this environment is omitted from the
env_list, so that it does not run by default and unexpectedly
overwrite documentation that may recently have been built before
changes are made that could cause generated documentation to be
different.