py-bitarray: updated to 2.7.2

2.7.2:
* speedup all count functionality by using `__builtin_popcountll` when
  available
* add `popcount64()` to `bitarray.h` - we assume now that `uint64_t` is
  always available
* improve testing

2.7.1:
* optimize `util.sc_encode()`

(adam)