Don't use Mapping for mutable dicts

Stop using Mapping where we actually mean Dict:
Mapping means "we only need a read-only dict" and most of the time
this is not really the case.

Additionally, fix one type hint where I forgot to add "Dict".

Signed-off-by: Martin Vrachev <mvrachev@vmware.com>

tuf/api/metadata.py

@@ -838,7 +838,7 @@ def __init__(
  self.unrecognized_fields = unrecognized_fields or {}
 
  @classmethod
- def from_dict(cls, delegations_dict: Mapping[str, Any]) -> "Delegations":
+ def from_dict(cls, delegations_dict: Dict[str, Any]) -> "Delegations":
  """Creates Delegations object from its dict representation."""
  keys = delegations_dict.pop("keys")
  keys_res = {}
@@ -900,7 +900,7 @@ def __init__(
  spec_version: str,
  expires: datetime,
  targets: Mapping[str, Any],
- delegations: Optional[Delegations] = None,
+ delegations: Optional[Dict[Delegations]] = None,
  unrecognized_fields: Optional[Mapping[str, Any]] = None,
  ) -> None:
  super().__init__(version, spec_version, expires, unrecognized_fields)