v3.1.1 release on PyPi

2021-03-25

• fix the span length calculation in explanation notebook; kudos @Ankush-Chander
• add BiasedTextRank by @Ankush-Chander (many thanks!)
• add conda environment.yml plus instructions
• use bandit to check for security issues
• use codespell to check for spelling errors
• add pre-commit checks in general
• update doc._.phrases in the call to change_focus() so the summarization will sync with the latest focus