py: Make form-feed character a space (following C isspace).

Eg, in CPython stdlib, email/header.py has a form-feed character.

py/unicode.c

@@ -22,7 +22,7 @@
 // table of attributes for ascii characters
 STATIC const uint8_t attr[] = {
     0, 0, 0, 0, 0, 0, 0, 0,
-    0, AT_SP, AT_SP, AT_SP, 0, AT_SP, 0, 0,
+    0, AT_SP, AT_SP, AT_SP, AT_SP, AT_SP, 0, 0,
     0, 0, 0, 0, 0, 0, 0, 0,
     0, 0, 0, 0, 0, 0, 0, 0,
     AT_SP, AT_PR, AT_PR, AT_PR, AT_PR, AT_PR, AT_PR, AT_PR,