Try to use kerberos-sspi if it is available

[brechtm]

On Windows, kerberos-sspi seems like a drop-in replacement for pykerberos.

[brechtm]

Though if I try to import kerberos_sspi as kerberos (in puresasl/mechanisms.py), I run into this:

C:\Users\bmachie\AppData\Local\Continuum\Miniconda3\envs\ml_irissearch\lib\site-packages\puresasl\mechanisms.py in __init__(self, sasl, p
rincipal, **props)
    444             _, self.context = kerberos.authGSSClientInit(service=krb_service,
--> 445                                                          principal=self.principal)
    446         except TypeError:

TypeError: authGSSClientInit() got an unexpected keyword argument 'principal'

Reported at kerberos-sspi: #5