Update doc8 min version
doc8 0.6.1 supports python till version 3.4 [1] which doesn't
fit the current runtime requirements of wallaby [2] i.e.
python3.6 and python3.8. This patch updates the minimum
version of doc8 to 0.8.1 which supports python3.6 [3].
[1]
[2]
[3]
openstackdocstheme>=2.2.0 # Apache-2.0
reno>=3.1.0 # Apache-2.0
sphinx>=2.0.0,!=2.1.0 # BSD
-doc8>=0.6.0 # Apache-2.0
+doc8>=0.8.1 # Apache-2.0
# The below is rewquired to build testing module reference
mock>=2.0.0 # BSD
