Fix crash when closing sessions with vendor-specific user types

[akaloti]

Fix #230

In the current version of this package, when using a custom user such as the Crypto User -- represented by value 0x80000001 -- on Thales Luna HSM 7, we run into problems when closing a session because:

• The Session.close() method calls Session.user_type(). (Both methods are defined in pkcs11/_pkcs11.pyx.)
• Session.user_type() then calls UserType().
• UserType(), defined in pkcs11/constants.py, crashes because the custom user value 0x80000001 does not match NOBODY (999), SO (0), or USER (1).

This PR fixes the problem by avoiding the indirect call to UserType() in Session.close(). It also updates Session.user_type() to not crash if the user type is a custom user.