Transform boto3.ServiceRequest to look like dynamic class

[PCManticore]

boto3.resource creates resources dynamically via a resource factory.
Unfortunately that completely breaks static analysis leading to spurious
false positives since pylint cannot determine sanely that attributes
exist or not.

Here's an example of accessing the Topic class out of the sns resource.
As you can see, the class is created dynamically rather than existing
in the codebase itself:

In [2]: boto3.resource
Out[2]: <function boto3.resource(*args, **kwargs)>

In [3]: boto3.resource('sns')
Out[3]: sns.ServiceResource()

In [4]: boto3.resource('sns').Topic
Out[4]: <bound method ResourceFactory._create_class_partial.<locals>.create_resource of sns.ServiceResource()>

This patch adds a fake __getattr__ method to ServiceRequest.
This will prevent pylint from emitting no-member at all for ServiceRequest
instances, but that is a good solution for now until we can load typeshed-like
annotation packages.

Close pylint-dev/pylint#3134