docs: Decouple website download version from package version

This prevents unreleased versions appearing on the website (e.g. 0.3.5.dev0),
but introduces the risk of forgetting to update the website after a release.
A better fix requires deeper design/workflow thought.

refs #1028

docs/conf.py

@@ -1,9 +1,8 @@
 import sys
 
-sys.path.append('..')
 sys.path.append('.')
-import mitogen
+
-VERSION = '.'.join(str(part) for part in mitogen.__version__)
+VERSION = '0.3.4'
 
 author = u'Network Genomics'
 copyright = u'2021, the Mitogen authors'