if 8 1 byte fields are included in a ctype field, it allows an extra byte to be included in the packing when there is no room left for the next field.

If I put the bitfields in a child structure then I get expected results.
 
In [35]: run ctypeSizeTest.py
Size is  4 Expected 3
0 0x10000 a0
0 0x10001 a1
0 0x10002 a2
0 0x10003 a3
0 0x10004 a4
0 0x10005 a5
0 0x10006 a6
0 0x10007 a7
0 0x40008 b0 <- Expected to be at offset 1, not 0.
2 0xc0000 b1 <- Expected to be at offset 1, not 2
Size is  3 Expected 3
0 0x1 a
1 0x40000 b0
1 0xc0004 b1

Does not happen on windows.

Error is in cfield.c

```
#ifndef MS_WIN32
    } else if (bitsize /* this is a bitfield request */
        && *pfield_size /* we have a bitfield open */
        && dict->size * 8 >= *pfield_size
        && (*pbitofs + bitsize) <= dict->size * 8) {
        /* expand bit field */
        fieldtype = EXPAND_BITFIELD;
#endif
```

Specifically dict->size * 8 >= *pfield_size
if *bitofs == *pfield_size then the current field is filled, and expanding the bitfield should not be done.

Consider adding this *pfield_size != *bitofs
#ifndef MS_WIN32
    } else if (bitsize /* this is a bitfield request */
        && *pfield_size /* we have a bitfield open */
        && *pfield_size != *pbitofs /* Current field has been filled, start new one */
        && dict->size * 8 >= *pfield_size
        && (*pbitofs + bitsize) <= dict->size * 8) {
        /* expand bit field */
        fieldtype = EXPAND_BITFIELD;
#endif