Support user abstract load shape base classes

[noirbizarre]

This PR improves the abstract custom base shape detection by using the same pattern than the base User class: abstract base classes should define the abstractattribute to True.

This change is entirely backward compatible and tested.

This PR fixes the following case:

• define a reusable LoadTestShape in your common modules (let's say lib.BaseShape)
• create a locust file, create a child shape (cf. below)
• run the bench

This will result in:

• without this change, the imported base class is picked as shape, the child class is ignored.
• with this change, the imported abstract base is ignored and the child class is selected as shape

lib.py

class BaseShape(LoadTestShape):
	abstract = True

	some_class_attr = None # to be provided by child implementation

	def tick(self):
        # Do something with `self.__class__.some_class_attr`

locusfile

from lib import BaseShape

class MyShape(BaseShape):
    some_class_attr = ["my test data"]

Currently, BaseShape will be chosen as shape_class and MyShape will be silently ignored (unless you specify it on the command line, or you rely on the class picker in which BaseShape will appear and shouldn't).

With this change, BaseShape is seen as abstract and is ignored from shape candidates, won't appear in the class picker and MyShape will be chosen if not explicitly told.

[cyberw]

Nice!