Pin pip to <19 until a more permanent solution can be devised. Ref #1644.

jaraco · jaraco · commit abf07f6598b1 · 2019-01-26T17:06:35.000-05:00
diff --git a/tests/requirements.txt b/tests/requirements.txt
@@ -1,3 +1,5 @@
+# workaround for #1644
+pip<19
 mock
 pytest-flake8; python_version!="3.4"
 pytest-flake8<=1.0.0; python_version=="3.4"
