Fix sentinel mode read-write separation

[JackiLin]

When I attempted to set up read-write separation in Sentinel mode by configuring redis://redis-master/db?is_master=0, I noticed that both read and write requests were still being directed to the master node, without any separation. After reviewing and debugging the source code, I found that the issue was due to redis-py being unable to correctly parse the is_master parameter, and there was also an error in how the project handled is_master. The pool.is_master was only set to False after retrieving the redis.sentinel.SentinelConnectionPool object from redis-py, which resulted in the system supposedly using the slave node, but actually using the master node.

[codecov]

Codecov Report

Attention: Patch coverage is 76.47059% with 4 lines in your changes missing coverage. Please review.

Project coverage is 63.3%. Comparing base (f34935c) to head (df4fd4d).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
django_redis/pool.py	66.7%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #749     +/-   ##
========================================
- Coverage    63.3%   63.3%   -0.0%     
========================================
  Files          43      43             
  Lines        3215    3221      +6     
  Branches      244     244             
========================================
+ Hits         2034    2037      +3     
- Misses       1164    1167      +3     
  Partials       17      17             

Flag	Coverage Δ
mypy	38.0% <17.7%> (-<0.1%)	⬇️
tests	90.3% <84.7%> (+0.1%)	⬆️

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