gdb/python: missing PyObject_IsTrue error check in micmdpy_set_installed

gdb/python: missing PyObject_IsTrue error check in micmdpy_set_installed

Like the previous commit, I discovered that in micmdpy_set_installed
we were calling PyObject_IsTrue, but not checking for a possible error
value being returned.

The micmdpy_set_installed function implements the
gdb.MICommand.installed attribute, and the documentation indicates that
this attribute should only be assigned a bool:

  This attribute is read-write, setting this attribute to 'False'
  will uninstall the command, removing it from the set of available
  commands.  Setting this attribute to 'True' will install the
  command for use.

So I propose that instead of using PyObject_IsTrue we use
PyBool_Check, and if the new value fails this check we raise an
error.  We can then compare the new value to Py_True directly instead
of calling PyObject_IsTrue.

This is a potentially breaking change to the Python API, but hopefully
this will not impact too many people, and the fix is pretty
easy (switch to using a bool).  I've added a NEWS entry to draw
attention to this change.

Approved-By: Tom Tromey <tom@tromey.com>