ffi/python: enable Seed.keygen(strength)

This is just another simple way to generate a key from seed based on
the strength of the desired object.

ffi/python/fips203.py

@@ -97,6 +97,7 @@
 
 Please report issues at https://github.com/integritychain/fips203/issues
 '''
+from __future__ import annotations
 
 '''__version__ should track package.version from  ../Cargo.toml'''
 __version__ = '0.4.0'
@@ -158,6 +159,12 @@ def __repr__(self) -> str:
     def __bytes__(self) -> bytes:
         return bytes(self._seed.data)
 
+    def keygen(self, strength: int) -> Tuple[EncapsulationKey, DecapsulationKey]:
+        for kt in ML_KEM_512, ML_KEM_768, ML_KEM_1024:
+            if kt._strength == strength:
+                return kt.keygen(self)
+        raise Exception(f"Unknown strength: {strength}, must be 512, 768, or 1024.")
+
 class Ciphertext():
     '''ML-KEM Ciphertext