* Adding aliases for field attributes and renaming async attribute
As of Python 3.7, the use of async raises an error, whereas before the use
of the reserved word was ignored. This adds an alias field for field attrs
so that both async and async_val (interally) work. This allows us to be
backwards-compatible with 3rd party plugins that may still reference Task.async,
but for the core engine to work on Py3.7+.
* Remove files fixed for 'async' usage from the python 3.7 skip list
Introduces the `inherit` param for FieldAttributes, which is now used
in BaseMeta when constructing the getter property to enhance performance
by reducing the amount of work the getter generally has to do.
moved from the field attribute declaration and created a placeholder
which then is resolved in the field attribute class.
this is to avoid unwanted persistent of the defaults across objects which introduces
stealth bugs when multiple objects of the same kind are used in succession while
not overriding the default values.
FieldAttributes will now by default not be post_validated unless a flag
is set on them in the class, as a large number of fields are really there
simply to be inherited by Task/PlayContext and shouldn't be templated too
early.
The other (unrelated to the base issue) in #12084 is also fixed here, where
the roles field is loaded before vars/vars_files, meaning there are no vars
yet loaded in the play when the templating occurs.
Fixes#12084
