Pin pip to 18.x (#559)

Author: ob-stripe

Makefile

@@ -2,7 +2,10 @@ init:
 	# Pin pipenv to 2018.10.13 because further releases are incompatible with
 	# PyPy3. The issue was fixed in https://github.com/pypa/pipenv/pull/3322
 	# but no new version has been released yet.
-	pip install --upgrade pipenv==2018.10.13
+	# Also pin pip to 18.x because with more recent versions, we'd need to
+	# pass `--no-use-pep517`, which pipenv doesn't let us do. Cf.
+	# https://github.com/pypa/pipenv/issues/3651
+	pip install --upgrade pip~=18.0 pipenv==2018.10.13
 	pipenv install --dev --skip-lock
 
 test: