use --pyproject

tox.ini

@@ -81,25 +81,11 @@ commands =
          python misc/coding_tools/find-trailing-spaces.py -r src static misc setup.py
          python misc/coding_tools/check-miscaptures.py
 
-	 # 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
-	 mv towncrier.pyproject.toml pyproject.toml
-
 	 # If towncrier.check fails, you forgot to add a towncrier news
 	 # fragment explaining the change in this branch.  Create one at
 	 # `newsfragments/<ticket>.<change type>` with some text for the news
 	 # file.  See pyproject.toml for legal <change type> values.
-	 python -m towncrier.check
-
-         # put it back
-	 mv pyproject.toml towncrier.pyproject.toml
+	 python -m towncrier.check --pyproject towncrier.pyproject.toml
 
 [testenv:draftnews]
 passenv = TAHOE_LAFS_* PIP_* SUBUNITREPORTER_* USERPROFILE HOMEDRIVE HOMEPATH
@@ -118,6 +104,9 @@ commands =
          #
          # 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