migrate python 3.5 compatible superclass variable annotations to 3.6

As we now require python 3.6, we can declare their types without
initializing them.

mesonbuild/compilers/compilers.py

@@ -479,11 +479,9 @@ class Compiler(HoldableObject, metaclass=abc.ABCMeta):
     LINKER_PREFIX = None  # type: T.Union[None, str, T.List[str]]
     INVOKES_LINKER = True
 
-    # TODO: these could be forward declarations once we drop 3.5 support
-    if T.TYPE_CHECKING:
-        language = 'unset'
-        id = ''
-        warn_args = {}  # type: T.Dict[str, T.List[str]]
+    language: str
+    id: str
+    warn_args: T.Dict[str, T.List[str]]
 
     def __init__(self, exelist: T.List[str], version: str,
                  for_machine: MachineChoice, info: 'MachineInfo',

mesonbuild/interpreterbase/decorators.py

@@ -551,9 +551,7 @@ def typed_kwargs(name: str, *types: KwargInfo) -> T.Callable[..., T.Any]:
 class FeatureCheckBase(metaclass=abc.ABCMeta):
     "Base class for feature version checks"
 
-    # In python 3.6 we can just forward declare this, but in 3.5 we can't
-    # This will be overwritten by the subclasses by necessity
-    feature_registry = {}  # type: T.ClassVar[T.Dict[str, T.Dict[str, T.Set[str]]]]
+    feature_registry: T.ClassVar[T.Dict[str, T.Dict[str, T.Set[str]]]]
 
     def __init__(self, feature_name: str, version: str, extra_message: T.Optional[str] = None):
         self.feature_name = feature_name  # type: str