[commands] Use eval instead of get_type_hints to resolve typehints

The previous usage of `typing.get_type_hints` caused issues as it would
incorrectly decide to convert annotations into their equivalent
`typing` form -- which is not what we want to happen here.

Due to some use-cases about how setting `Command.callback` work and
the amount of moving parts that have changed due to this patch, it is
probably better to refactor the way it is set so users can have this
use-case handled transparently for them.

discord/ext/commands/core.py

@@ -174,21 +174,7 @@ class Command:
         self.description = inspect.cleandoc(kwargs.get('description', ''))
         self.hidden = kwargs.get('hidden', False)
 
-        signature = inspect.signature(callback)
-        annotations = typing.get_type_hints(callback)
-
-        self.params = signature.parameters.copy()
-
-        # PEP-563 allows postponing evaluation of annotations with a __future__
-        # import. When postponed, Parameter.annotation will be a string and must
-        # be replaced with the real class from typing.get_type_hints() for the
-        # converters to work later on
-        for key, value in self.params.items():
-            if isinstance(value.annotation, str) and key in annotations:
-                self.params[key] = value.replace(annotation=annotations[key])
-
         self.checks = kwargs.get('checks', [])
-        self.module = callback.__module__
         self.ignore_extra = kwargs.get('ignore_extra', True)
         self.instance = None
         self.parent = None
@@ -196,6 +182,25 @@ class Command:
         self._before_invoke = None
         self._after_invoke = None
 
+    @property
+    def callback(self):
+        return self._callback
+
+    @callback.setter
+    def callback(self, function):
+        self._callback = function
+        self.module = function.__module__
+
+        signature = inspect.signature(function)
+        self.params = signature.parameters.copy()
+
+        # PEP-563 allows postponing evaluation of annotations with a __future__
+        # import. When postponed, Parameter.annotation will be a string and must
+        # be replaced with the real value for the converters to work later on
+        for key, value in self.params.items():
+            if isinstance(value.annotation, str):
+                self.params[key] = value.replace(annotation=eval(value.annotation, function.__globals__))
+
     async def dispatch_error(self, ctx, error):
         ctx.command_failed = True
         cog = self.instance