Pip 72 was released yesterday (2024-07-28), dropping `setup.py test` support.
hdrhistogram 0.6.1 requires it to install.
For now constrain Pip to earlier releases, so our tests can be run.
refs #1090
The Travis Linux distribution must be upgraded because the Trusty
(Ubuntu 14.04) image does not have Python 3.9. Xenial (Ubuntu 16.04) is
the earliest version that offers Python 3.9.
I have not chosen a later release, in order to aid restoration of Python
2.4 - 2.6 tests.
Alex Willmer