* Improve exception compliance with PEP 249
* Raise InterfaceError instead of ProgrammingError for SQLITE_MISUSE.
If SQLITE_MISUSE is raised, it is a sqlite3 module bug. Users of the
sqlite3 module are not responsible for using the SQLite C API correctly.
* Don't overwrite BufferError with ValueError when conversion to BLOB fails.
* Raise ProgrammingError instead of Warning if user tries to execute() more
than one SQL statement.
* Raise ProgrammingError instead of ValueError if an SQL query contains null characters.
* Make sure `_pysqlite_set_result` raises an exception if it returns -1.
