text: Title: Constant for SQLITE_INDEX_CONSTRAINT_ISNULL is Incorrect
  Body: Right now the constant for `SQLITE_INDEX_CONSTRAINT_ISNULL` is hardcoded `69` https://github.com/betodealmeida/shillelagh/blob/14579e4b8c3159adc4076b36638d13f00dc70609/src/shillelagh/backends/apsw/vt.py#L68-L69
  however that value should be `71`. See https://www.sqlite.org/c3ref/c_index_constraint_eq.html
  
  The ordering here is not what determines the constant value: https://github.com/rogerbinns/apsw/blob/f2649fc7b0f7e80a5f18fa60ae796d6fd9aaf214/src/apsw.c#L1531-L1533
  
  This can all be confirmed in Python:
  ```
  >>> apsw.SQLITE_INDEX_CONSTRAINT_ISNULL
  71
  ```