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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[ivorpeles]

Ran into this as well, would really like to see this merged. Can further confirm that if you pass in instances mypy is satisfied but my code breaks.

Even in the test code for arq the correct types are passed, even though this isn't detected by CI:
https://github.com/samuelcolvin/arq/blob/1315583f170512fc63622af0200a2855e057a58d/tests/conftest.py#L58