Fix it only one way

MyPy can't handle a Pythonic try-except import: python/mypy#1393
We could put # type: ignore[attr-defined] except then whichever branch
actually passed on the currently installed version of the module would
fail due to having an apparently-unneeded ignore.

src/toil/lib/aws/iam.py

@@ -7,12 +7,7 @@
 
 import boto3
 from mypy_boto3_iam import IAMClient
-try:
-    from mypy_boto3_iam.type_defs import AttachedPolicyTypeDef
-except ImportError:
-    # mypy_boto3_iam==1.28.3.post1 shipped this under a different name. It
-    # might move again in the future?
-    from mypy_boto3_iam.type_defs import AttachedPolicyOutputTypeDef as AttachedPolicyTypeDef
+from mypy_boto3_iam.type_defs import AttachedPolicyTypeDef
 from mypy_boto3_sts import STSClient
 
 from toil.lib.aws import zone_to_region