py2bit.c: fix pointer bug flagged by gcc-14.

[emollier]

Since gcc 14, a whole range of warnings have become errors, notably the casting from incompatible pointer types. In the case of py2bit, this seems to flag a real pointer arithmetic bug. The symptom on build time is:

py2bit.c:35:24: error: initialization of ‘pyTwoBit_t *’ from incompatible pointer type ‘TwoBit *’ [-Wincompatible-pointer-types]
   35 |     pyTwoBit_t *pytb = self->tb;
      |                        ^~~~

Transmitting the self pointer straight to the pytb pointer newly created without referencing to the TwoBit_t pointer fixes the issue.

This was initially reported under Debian bug #1075421 by Matthias Klose.