Stop using formats.py in metadata.py

Currently we have one use of tuf/formats.py in tuf/api/metadata.py.
If we do the conversion of the expires string in metadata.py,
we can keep the two implementations separate.

Signed-off-by: Velichka Atanasova <avelichka@vmware.com>

Author: avelichka

tuf/api/metadata.py

@@ -25,7 +25,7 @@
 from securesystemslib.storage import FilesystemBackend, StorageBackendInterface
 from securesystemslib.util import persist_temp_file
 
-from tuf import exceptions, formats
+from tuf import exceptions
 from tuf.api.serialization import (
     MetadataDeserializer,
     MetadataSerializer,
@@ -384,7 +384,7 @@ def _common_fields_from_dict(
         # Convert 'expires' TUF metadata string to a datetime object, which is
         # what the constructor expects and what we store. The inverse operation
         # is implemented in '_common_fields_to_dict'.
-        expires = formats.expiry_string_to_datetime(expires_str)
+        expires = datetime.strptime(expires_str, "%Y-%m-%dT%H:%M:%SZ")
         return version, spec_version, expires
 
     def _common_fields_to_dict(self) -> Dict[str, Any]: