Validate spec_version during initialization

Even though version strings like "2.0.0-rc.2" or "1.0.0-beta" are
valid strings in semantic versioning format, in TUF we never needed
to add letters for our specification number.

That's why I validate that: spec_version is a . separated string
and when split it has a length of 3 and that each of the
three elements is a number.

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

Author: Martin Vrachev

tuf/api/metadata.py

@@ -343,6 +343,11 @@ def __init__(
         expires: datetime,
         unrecognized_fields: Optional[Mapping[str, Any]] = None,
     ) -> None:
+        spec_list = spec_version.split(".")
+        if len(spec_list) != 3 or not all(el.isdigit() for el in spec_list):
+            raise ValueError(
+                f"spec_version must be in semver format, got {spec_version}"
+            )
         self.spec_version = spec_version
         self.expires = expires