fix in-comment Sage code to work with Sage9.0+ (based on Python3)

Some of the C source files include Sage code in comments. This PR
updates these to work with a current version based on Python3 (Sage
9.0+, see https://wiki.sagemath.org/Python3-Switch).

This can be seen as a small follow-up to PR #849 (commit
13c88ef).

src/ecdsa_impl.h

@@ -16,10 +16,10 @@
 #include "ecdsa.h"
 
 /** Group order for secp256k1 defined as 'n' in "Standards for Efficient Cryptography" (SEC2) 2.7.1
- *  sage: for t in xrange(1023, -1, -1):
+ *  sage: for t in range(1023, -1, -1):
  *     ..   p = 2**256 - 2**32 - t
  *     ..   if p.is_prime():
- *     ..     print '%x'%p
+ *     ..     print('%x'%p)
  *     ..     break
  *   'fffffffffffffffffffffffffffffffffffffffffffffffffffffffefffffc2f'
  *  sage: a = 0

src/tests.c

@@ -4022,9 +4022,9 @@ static void test_add_neg_y_diff_x(void) {
      * # random "bad pair"
      * P = C.random_element()
      * Q = -int(lam) * P
-     * print "    P: %x %x" % P.xy()
-     * print "    Q: %x %x" % Q.xy()
-     * print "P + Q: %x %x" % (P + Q).xy()
+     * print("    P: %x %x" % P.xy())
+     * print("    Q: %x %x" % Q.xy())
+     * print("P + Q: %x %x" % (P + Q).xy())
      */
     secp256k1_gej aj = SECP256K1_GEJ_CONST(
         0x8d24cd95, 0x0a355af1, 0x3c543505, 0x44238d30,