Add _ext.py for Python codegen

[emilk]

We currently rely on .ovverrides to inject code at runtime.

Sometimes it is much cleaner to inject code at compile-time. For instance:

setattr(TextLogLevelType, "CRITICAL", "CRITICAL")
TextLogLevelType.CRITICAL.__doc__ = """ Designates catastrophic failures. """

setattr(TextLogLevelType, "ERROR", "ERROR")
TextLogLevelType.ERROR.__doc__ = """ Designates very serious errors. """

setattr(TextLogLevelType, "WARN", "WARN")
TextLogLevelType.WARN.__doc__ = """ Designates hazardous situations. """

setattr(TextLogLevelType, "INFO", "INFO")
TextLogLevelType.INFO.__doc__ = """ Designates useful information. """

setattr(TextLogLevelType, "DEBUG", "DEBUG")
TextLogLevelType.DEBUG.__doc__ = """ Designates lower priority information. """

setattr(TextLogLevelType, "TRACE", "TRACE")
TextLogLevelType.TRACE.__doc__ = """ Designates very low priority, often extremely verbose, information. """

is pretty ugly, and I'm not even sure it works. It would be much nicer to output this code:

class TextLogLevelType(BaseDelegatingExtensionType):
    CRITICAL: Final = "CRITICAL"
    """ Designates catastrophic failures. """

    ERROR: Final = "ERROR"
    """ Designates very serious errors. """

    WARN: Final = "WARN"
    """ Designates hazardous situations. """

    INFO: Final = "INFO"
    """ Designates useful information. """

    DEBUG: Final = "DEBUG"
    """ Designates lower priority information. """

    TRACE: Final = "TRACE"
    """ Designates very low priority, often extremely verbose, information. """

    # codegen code here

[emilk]

We also want this in order to implement Boxes2D::from_xywh etc

[emilk]

@jleibs says we could be using "mixin classes" for this