Update Jinja2 to 2.8.1, includes a security fix.

Fixes #4193

requirements/compiled.txt

@@ -1,8 +1,7 @@
-
 # Jinja2 is required by Sphinx
-Jinja2==2.8 \
-    --hash=sha256:1cc03ef32b64be19e0a5b54578dd790906a34943fe9102cfdae0d4495bd536b4 \
-    --hash=sha256:bc1ff2ff88dbfacefde4ddde471d1417d3b304e8df103a7a9437d47269201bf4
+Jinja2==2.8.1 \
+    --hash=sha256:3997cf273f1424207c60d5895264f74483fce72702f15a7cd51a8551d43663ca \
+    --hash=sha256:35341f3a97b46327b3ef1eb624aadea87a535b8f50863036e085e7c426ac5891
 # MarkupSafe is required by Jinja2
 MarkupSafe==0.23 \
     --hash=sha256:a4ec1aff59b95a14b45eb2e23761a0179e98319da5a7eb76b56ea8cdc7b871c3