py/formatfloat: Fix case where floats could render with a ":" character.

Prior to this patch, some architectures (eg unix x86) could render floats
with a ":" character in them, eg 1e+39 would come out as ":e+38" (":" is
just after "9" in ASCII so this is like 10e+38).  This patch fixes some of
these cases.

py/formatfloat.c

@@ -258,7 +258,7 @@ int mp_format_float(FPTYPE f, char *buf, size_t buf_size, char fmt, int prec, ch
         }
 
         // It can be that f was right on the edge of an entry in pos_pow needs to be reduced
-        if (f >= FPCONST(10.0)) {
+        if ((int)f >= 10) {
             e += 1;
             f *= FPCONST(0.1);
         }

tests/float/float_format.py

@@ -9,3 +9,7 @@ for val in (116, 1111, 1234, 5010, 11111):
 # make sure rounding is done at the correct precision
 for prec in range(8):
     print(('%%.%df' % prec) % 6e-5)
+
+# check certain cases that had a digit value of 10 render as a ":" character
+print('%.2e' % float('9' * 51 + 'e-39'))
+print('%.2e' % float('9' * 40 + 'e-21'))