Merge BlockstreamResearch/secp256k1-zkp#292: doc: fix sage code for deriving alternative generator H

7040a20247 doc: fix sage code for deriving alternative generator H (Sebastian Falbesoner)

Pull request description:

  The line calculating H (in particular, the expression `G.decode('hex')`) fails with the following error message on Sage 9.5:

  ```
  AttributeError: 'str' object has no attribute 'decode'
  ```

  Fix that by converting the hex-string to bytes using `bytes.fromhex`.

  (Noticed while reviewing https://github.com/bitcoin/bitcoin/pull/30048 which picks this code snippet comment up.)

ACKs for top commit:
  josibake:
    ACK 7040a20247
  real-or-random:
    utACK 7040a20247

Tree-SHA512: 0a44f399b103c2f5840056d163c1483a1d4f032bc0f8d3822507ac6da9d567f46e36caa79c7f5016aebcc8827b79e9aec7ebdb4f21c3c0242dc6875be140f289

src/modules/generator/main_impl.h

@@ -24,7 +24,7 @@
     import hashlib
     F = FiniteField (0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEFFFFFC2F)
     G = '0479be667ef9dcbbac55a06295ce870b07029bfcdb2dce28d959f2815b16f81798483ada7726a3c4655da4fbfc0e1108a8fd17b448a68554199c47d08ffb10d4b8'
-    H = EllipticCurve ([F (0), F (7)]).lift_x(F(int(hashlib.sha256(G.decode('hex')).hexdigest(),16)))
+    H = EllipticCurve ([F (0), F (7)]).lift_x(F(int(hashlib.sha256(bytes.fromhex(G)).hexdigest(),16)))
     print('%x %x' % H.xy())
  */
 static const secp256k1_generator secp256k1_generator_h_internal = {{