Fix #212: Pin Redis to version 2.10.6

[ericof]

Version 3.0 of Redis breaks Celery, so we need to pin to version 2.10.6

Redis 3.0 release note:

• https://github.com/andymccurdy/redis-py/blob/3.0/README.rst

Celery incompatibility with Redis 3.0:

• AttributeError: 'float' object has no attribute 'items' celery/celery#5175

[codecov-io]

Codecov Report

Merging #214 into master will increase coverage by 9.69%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #214      +/-   ##
==========================================
+ Coverage   67.69%   77.38%   +9.69%     
==========================================
  Files         132      132              
  Lines        6636     5944     -692     
  Branches      768      632     -136     
==========================================
+ Hits         4492     4600     +108     
+ Misses       1863     1102     -761     
+ Partials      281      242      -39

Impacted Files	Coverage Δ
websauna/system/core/redis.py	92.72% <100%> (+1.81%)	⬆️
websauna/utils/autoevent.py	50.9% <0%> (-1.82%)	⬇️
websauna/system/task/celeryloader.py	0% <0%> (ø)	⬆️
websauna/system/model/sqlalchemyutcdatetime.py	0% <0%> (ø)	⬆️
websauna/system/user/googleoauth.py	0% <0%> (ø)	⬆️
websauna/system/user/userregistry.py	92.37% <0%> (+0.84%)	⬆️
websauna/system/user/oauthloginservice.py	36.48% <0%> (+1.35%)	⬆️
websauna/system/user/loginservice.py	94.44% <0%> (+1.38%)	⬆️
websauna/system/model/retry.py	74.6% <0%> (+1.58%)	⬆️
websauna/system/user/social.py	36.73% <0%> (+1.73%)	⬆️
... and 60 more

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