DOC: Set doc build CI's epoch to date of last tag.

This should reduce the differences between doc builds a little bit, and
though we do throw away commits to update devdocs, it might save a
little bit in git internally.

Author: QuLogic

.circleci/config.yml

@@ -63,6 +63,8 @@ mpl-run: &mpl-install
 doc-run: &doc-build
   name: Build documentation
   command: |
+    # Set epoch to date of latest tag.
+    export SOURCE_DATE_EPOCH="$(git log -1 --format=%at $(git describe --abbrev=0))"
     make html O=-T
     rm -r build/html/_sources
   working_directory: doc