* Add a new, simplified version of pipstrap.
* Use tools/pipstrap.py
* Uncomment code
* Refactor pip_install.py and provide hashes.
* Fix test_sdists.sh.
* Make code work on Python 2.
* Call strip_hashes.py using Python 3.
* Pin the oldest version of httplib2 used in distros
* Strip enum34 dependency.
* Remove pip pinnings from dev_constraints.txt
* Correct pipstrap docstring.
* Don't set working_dir twice.
* Add comments
Fixes#8071 and fixes https://github.com/certbot/certbot/issues/8110.
This PR migrates every job from Travis in Azure Pipeline.
This PR essentially converts the Travis jobs into Azure Pipeline with a complete iso-fonctionality (or I made a mistake). The jobs are added in the relevant existing pipelines (`main`, `nightly`, `advanced-test`, `release`). A global refactoring thanks to the templating system is done to reduce greatly the verbosity of the pipeline descriptions.
A specific feature (not present in Travis) is added: the stage `On_Failure`. Using directly the Mattermost API, it allows to notify pipeline failure in a Mattermost channel with a link to the failed pipelines without the need to authenticate to Microsoft.
See https://github.com/certbot/certbot/pull/8098#issuecomment-649873641 for the post merge actions to do at the end of this work.
