Metadata API: fix Key validation exception

In pr #1449 we introduced
Key validation and there decided to raise "ValueError" if one of
keyid, scheme, keyval or keytype is not a string.
That seems like a mistake given that in the python docs it's written:
"Passing arguments of the wrong type
(e.g. passing a list when an int is expected) should result
in a TypeError, but passing arguments with the wrong value
(e.g. a number outside expected boundaries) should result
in a ValueError."

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

Author: MVrachev

tuf/api/metadata.py

@@ -427,7 +427,7 @@ def __init__(
         if not all(
             isinstance(at, str) for at in [keyid, keytype, scheme]
         ) or not isinstance(keyval, Dict):
-            raise ValueError("Unexpected Key attributes types!")
+            raise TypeError("Unexpected Key attributes types!")
         self.keyid = keyid
         self.keytype = keytype
         self.scheme = scheme