Revert "docs: Increase support range for reno"

This reverts commit 996c097. Turns out
we were trying to build docs with Python 2 instead of Python 3. This is
resolved now.

Signed-off-by: Stephen Finucane <stephen@that.guru>

Author: stephenfin

docs/requirements.txt

@@ -1,5 +1,5 @@
 -r ../requirements.txt
 sphinx>=1.5,<2.0
 sphinx-click>=2.0,<3.0
-reno>=2.0,<4.0
+reno>=3.0,<4.0
 sphinx-rtd-theme==0.4.3