extmod/modujson: Handle parsing of floats with + in the exponent.

Fixes issue #4780.

extmod/modujson.c

@@ -185,7 +185,7 @@ STATIC mp_obj_t mod_ujson_load(mp_obj_t stream_obj) {
                     cur = S_CUR(s);
                     if (cur == '.' || cur == 'E' || cur == 'e') {
                         flt = true;
-                    } else if (cur == '-' || unichar_isdigit(cur)) {
+                    } else if (cur == '+' || cur == '-' || unichar_isdigit(cur)) {
                         // pass
                     } else {
                         break;

tests/extmod/ujson_loads_float.py

@@ -14,4 +14,5 @@ my_print(json.loads('1.2'))
 my_print(json.loads('1e2'))
 my_print(json.loads('-2.3'))
 my_print(json.loads('-2e3'))
+my_print(json.loads('-2e+3'))
 my_print(json.loads('-2e-3'))