use Tox to build NEWS

tox.ini

@@ -115,6 +115,37 @@ commands =
          # put it back
          mv pyproject.toml towncrier.pyproject.toml
 
+[testenv:news]
+passenv = TAHOE_LAFS_* PIP_* SUBUNITREPORTER_* USERPROFILE HOMEDRIVE HOMEPATH
+# see comment in [testenv] about "certifi"
+whitelist_externals = mv
+deps =
+     certifi
+     towncrier >= 19.2
+commands =
+         # With pip >= 10 the existence of pyproject.toml (which we are
+         # required to have to configure towncrier) triggers a "build
+         # isolation" mode which prevents anything from working.  Avoid
+         # triggering that pip behavior by keeping the towncrier configuration
+         # somewhere else and only bringing it in when it's actually needed
+         # (after pip is done).
+         #
+         # Some discussion is available at
+         # https://github.com/pypa/pip/issues/5696
+	 #
+	 # towncrier post 19.2 (unreleased as of this writing) adds a --config
+	 # option that can be used instead of this file shuffling.
+         mv towncrier.pyproject.toml pyproject.toml
+
+         # towncrier 19.2 + works with python2.7
+         python -m towncrier --yes
+
+         # put it back
+         mv pyproject.toml towncrier.pyproject.toml
+
+         # commit the changes
+         git commit -m "update NEWS.txt for release"
+
 [testenv:deprecations]
 setenv =
          PYTHONWARNINGS=default::DeprecationWarning