Merge pull request #1825 from MVrachev/specification-version

Update supported specification version to 1.0.28

Author: Jussi Kukkonen

tuf/api/metadata.py

@@ -74,7 +74,7 @@
 
 # We aim to support SPECIFICATION_VERSION and require the input metadata
 # files to have the same major version (the first number) as ours.
-SPECIFICATION_VERSION = ["1", "0", "19"]
+SPECIFICATION_VERSION = ["1", "0", "28"]
 TOP_LEVEL_ROLE_NAMES = {_ROOT, _TIMESTAMP, _SNAPSHOT, _TARGETS}
 
 # T is a Generic type constraint for Metadata.signed