[10102] Add towncriertox env.

[adiroiban]

Scope and purpose

This adds a towncrie tox env to make it a bit easier to create the news fragment.

Another option , is to update the release docs not to required using tox to call towncrier.

Contributor Checklist:

• The associated ticket in Trac is here: https://twistedmatrix.com/trac/ticket/10102
• I ran tox -e lint to format my patch to meet the Twisted Coding Standard
• I have created a newsfragment in src/twisted/newsfragments/ (see: News files)
• [NA] I have updated the automated tests.
• I have submitted the associated Trac ticket for review by adding the word review to the keywords field in Trac, and putting a link to this PR in the comment; it shows up in https://twisted.reviews/ now.

[rodrigc]

This looks OK. This is too late to make it into the 21.2.0 release,
but it is not a release blocker, since I can run towncrier outside of tox when doing the release.

[adiroiban]

Thanks for the review. No problem if this is not included in the release.