Improper verifying of param: "exceptions" in retry

[gryznar]

In function retry there is auto conversion to tuple if parametr exceptions is instance of Exception:

if isinstance(exceptions, Exception):  # pragma: no cover
    exceptions = (exceptions,) 

But this is improper from at least 2 reasons:

1. Documentation of this param says:
   """
   exceptions (tuple, optional): Tuple of exceptions that trigger a retry attempt. Exceptions
   not in the tuple will be ignored. Defaults to (Exception,) (all exceptions).
   """ <- single Exception is not mentioned here
2. Check itself. Should be:

if issubclass(exceptions, Exception):  # pragma: no cover
    exceptions = (exceptions,) 

How it could be fixed:

1. Fix check and update documentation to mention single exception class in possible values
   OR
2. Remove this check and conversion

[Derrick Gilland]

Thanks for reporting!

I opted to just remove the check/conversion since it wasn't working and not documented to work anyway.