Simplifications on hashed_bin_delegatio example

We no longer need or use SPEC_VERSION variable defined in the
begging of the script.

Additionally, I decided to add a small addition to the "roles"
type annotation as that gives better context to the syntax highlighter
of VS code.

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

Author: Martin Vrachev

examples/repo_example/hashed_bin_delegation.py

@@ -26,7 +26,6 @@
 from securesystemslib.signer import SSlibSigner
 
 from tuf.api.metadata import (
-    SPECIFICATION_VERSION,
     DelegatedRole,
     Delegations,
     Key,
@@ -42,8 +41,7 @@ def _in(days: float) -> datetime:
     return datetime.utcnow().replace(microsecond=0) + timedelta(days=days)
 
 
-SPEC_VERSION = ".".join(SPECIFICATION_VERSION)
-roles: Dict[str, Metadata] = {}
+roles: Dict[str, Metadata[Targets]] = {}
 keys: Dict[str, Dict[str, Any]] = {}
 
 # Hash bin delegation