Revert "bump pyyaml to 4.1"

This reverts commit 1de29c4.

The 4.1 release of pyaml was dropped from pypi because of breakages similar to what we had to fix in 94d646e (which I'm keeping). See yaml/pyyaml#193.

requirements.txt

@@ -251,8 +251,15 @@ pytz==2018.4 \
     --hash=sha256:65ae0c8101309c45772196b21b74c46b2e5d11b6275c45d251b150d5da334555 \
     --hash=sha256:c06425302f2cf668f1bba7a0a03f3c1d34d4ebeef2c72003da308b3947c7f749 \
     # via django, django-notifications-hq, fs, tzlocal
-pyyaml==4.1 \
-    --hash=sha256:a9b322285f419429402d5ce15e8f6d5c7a64f659eb87251af8b35c106f00a8e3 \
+pyyaml==3.12 \
+    --hash=sha256:16b20e970597e051997d90dc2cddc713a2876c47e3d92d59ee198700c5427736 \
+    --hash=sha256:3262c96a1ca437e7e4763e2843746588a965426550f3797a79fca9c6199c431f \
+    --hash=sha256:592766c6303207a20efc445587778322d7f73b161bd994f227adaa341ba212ab \
+    --hash=sha256:5ac82e411044fb129bae5cfbeb3ba626acb2af31a8d17d175004b70862a741a7 \
+    --hash=sha256:827dc04b8fa7d07c44de11fabbc888e627fa8293b695e0f99cb544fdfa1bf0d1 \
+    --hash=sha256:bc6bced57f826ca7cb5125a10b23fd0f2fff3b7c4701d64c439a300ce665fff8 \
+    --hash=sha256:c01b880ec30b5a6e6aa67b09a2fe3fb30473008c85cd6a67359a1b15ed6d83a4 \
+    --hash=sha256:e863072cdf4c72eebf179342c94e6989c67185842d9997960b3e69290b2fa269 \
     # via bandit, dparse, rtyaml
 requests-oauthlib==1.0.0 \
     --hash=sha256:8886bfec5ad7afb391ed5443b1f697c6f4ae98d0e5620839d8b4499c032ada3f \