A different work-around for the pep 518 mess

tox.ini

@@ -14,13 +14,6 @@ skipsdist = True
 [testenv]
 basepython=python2.7
 passenv = TAHOE_LAFS_* PIP_* SUBUNITREPORTER_* USERPROFILE HOMEDRIVE HOMEPATH
-setenv =
-       # 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.  Un-trigger this pip behavior
-       # so that packages pre-installed in a virtualenv are available.
-       PIP_NO_BUILD_ISOLATION=1
-
 # Get "certifi" to avoid bug #2913. Basically if a `setup_requires=...` causes
 # a package to be installed (with setuptools) then it'll fail on certain
 # platforms (travis's OX-X 10.12, Slackware 14.2) because PyPI's TLS
@@ -57,11 +50,23 @@ commands =
          python misc/coding_tools/check-debugging.py
          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
+	 python -m towncrier.check
 
 [testenv:deprecations]
 setenv =