purity theme, added virtualenv name display

[vlanx]

Description

Added a function to show the name of the python virtualenv, when its being used.

Motivation and Context

I consider having the name of the virtual environment we're in very useful.

How Has This Been Tested?

Entering and leaving a virtualenv has the desired behaviour.

Screenshots (if appropriate):

Demo

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• If my change requires a change to the documentation, I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• If I have added a new file, I also added it to clean_files.txt and formatted it using lint_clean_files.sh.
• I have added tests to cover my changes, and all the new and existing tests pass.

[vlanx]

If it's approved, will follow with a clean-up PR

[davidpfarrell]

Many of the themes have their own venv detection logic, so adding one here is reasonable.

However, the detect_venv; PS1+=... approach seems outdated (although still used by some themes)

I suggest just echoing the value from within your method, leaving your PS1 definition more clean.

NOTE: I also recommend renaming detect_venv => venv_prompt

[davidpfarrell]

I have one small comment but I think this is almost ready to go !

[davidpfarrell]

If it's approved, will follow with a clean-up PR

It occurs to me that now might be a good time to add themes/purity to the themes section of clean_files.txt.

This will allow our internal lint-checkers to run against the theme.

[davidpfarrell]

This seems great to me !

[NoahGorny]

Well done @tiagobarros99 and great review @davidpfarrell 😄