Fix "retry_on_error" type annotation

[armicron]

When a list of classes (not instances) is assigned to RedisSettings.retry_on_error, mypy finds an error:

$ make mypy
mypy arq
arq/poc_annotations.py:5: error: List item 0 has incompatible type "type[ConnectionError]"; expected "Exception"  [list-item]
Found 1 error in 1 file (checked 13 source files)
make: *** [Makefile:46: mypy] Error 1

Redis library for the similar parameter has the type annotation Optional[List[Type[Exception]]] (source)

How to reproduce:
create a file arq/poc_annotations.py

from arq.connections import RedisSettings
from redis.exceptions import ConnectionError


settings = RedisSettings(retry_on_error=[ConnectionError])

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.47%. Comparing base (94cd878) to head (ae148a0).
Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #446      +/-   ##
==========================================
+ Coverage   96.27%   96.47%   +0.19%     
==========================================
  Files          11       11              
  Lines        1074     1078       +4     
  Branches      209      190      -19     
==========================================
+ Hits         1034     1040       +6     
  Misses         19       19              
+ Partials       21       19       -2     

Files	Coverage Δ
arq/connections.py	89.93% <100.00%> (-0.13%)	⬇️

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