python: Pin fixtures dep to v4.0.0

Upstream "fixtures" python package is currently half-pushed or otherwise
broken.

Change-Id: I7059ed836d7a52c5ba587ad73cc5e4b21cf84887

python-requirements.txt

diff --git a/python-requirements.txt b/python-requirements.txt
index 33538a6..94de1ba 100644
--- a/python-requirements.txt
+++ b/python-requirements.txt
@@ -1,3 +1,7 @@
+# pin to v4.0.0 since newer pushes are broken
+# TODO(jtgans): Remove this pinning once upstream is unborked.
+fixtures==4.0.0
+
 camkes-deps
 hjson
 lit