Pin publicsuffix2 version

Something strange is going on with this package - after no updates for 2
years it's suddenly had two new versions released without having its
changelog updated, and the new versions don't seem to work properly. You
can no longer import it as "publicsuffix" like you used to be able to,
and it appears that the newest version (2.20180921.2) no longer even
downloads the publicsuffix file, it just uses the one included in the
package.

tildes/requirements-to-freeze.txt

@@ -16,7 +16,7 @@ Pillow
 prometheus-client
 prospector
 psycopg2
-publicsuffix2
+publicsuffix2==2.20160818
 Pygments
 pyotp
 pyramid