py/parsenum: Fix parsing of complex "j" and also "nanj", "infj".

Prior to this commit, complex("j") would return 0j, and complex("nanj")
would return nan+0j.  This commit makes sure "j" is tested for after
parsing the number (nan, inf or a decimal), and also supports the case of
"j" on its own.

Signed-off-by: Damien George <damien@micropython.org>

py/parsenum.c

@@ -291,9 +291,6 @@ parse_start:
                 if (str == top) {
                     goto value_error;
                 }
-            } else if (allow_imag && (dig | 0x20) == 'j') {
-                real_imag_state |= REAL_IMAG_STATE_HAVE_IMAG;
-                break;
             } else if (dig == '_') {
                 continue;
             } else {
@@ -327,6 +324,15 @@ parse_start:
         }
     }
 
+    if (allow_imag && str < top && (*str | 0x20) == 'j') {
+        if (str == str_val_start) {
+            // Convert "j" to "1j".
+            dec_val = 1;
+        }
+        ++str;
+        real_imag_state |= REAL_IMAG_STATE_HAVE_IMAG;
+    }
+
     // negate value if needed
     if (dec_neg) {
         dec_val = -dec_val;