Make Controller_T covariant to allow overridding @command methods

e.g.
/workspaces/fastcs-eiger/src/fastcs_eiger/controllers/odin/eiger_odin_controller.py:28:15 - error: "arm_when_ready" overrides symbol of same name in class "EigerController"
    "UnboundCommand[EigerOdinController]" is not assignable to "UnboundCommand[EigerController]"
        Type parameter "Controller_T@UnboundCommand" is invariant, but "EigerOdinController" is not the same as "EigerController" (reportIncompatibleVariableOverride)

Author: GDYendell

src/fastcs/methods/method.py

@@ -8,7 +8,7 @@
 
 MethodCallback = Callable[..., Coroutine[None, None, None]]
 """Generic protocol for all `Controller` Method callbacks"""
-Controller_T = TypeVar("Controller_T", bound=BaseController)
+Controller_T = TypeVar("Controller_T", bound=BaseController, covariant=True)
 """Generic `Controller` class that an unbound method must be called with as `self`"""