py: Fix undefined left shift operations.

By ensuring the value to be shifted is an unsigned of the appropriate type.

This fixes several runtime diagnostics such as:

    ../../py/binary.c:199:28: runtime error:
     left shift of 32768 by 16 places
     cannot be represented in type 'int'

Signed-off-by: Jeff Epler <jepler@gmail.com>

py/binary.c

@@ -196,7 +196,7 @@ static float mp_decode_half_float(uint16_t hf) {
         ++e;
     }
 
-    fpu.i = ((hf & 0x8000) << 16) | (e << 23) | (m << 13);
+    fpu.i = ((hf & 0x8000u) << 16) | (e << 23) | (m << 13);
     return fpu.f;
 }