Add "Mac OS X" deviceClass condition

[jetfir3]

• this properly addresses the issue first addressed in https://github.com/beeper/mac-registration-provider/pull/11 when using macOS devices prior to Big Sur

As is, Beeper Mini will crash if trying to register with mac-registration-provider when running macOS versions before v11.x as the sw_ver output provides Mac OS X and not macOS. It fails during the "generate albert push certificate" step.

It appears that macOS versions prior to macOS 11 still report the deviceClass as MacOS. For reference, an ActivationInfoXML from OS X 10.7.3 on https://kb.imfreedom.org/protocols/imessage/

...

    <key>DeviceClass</key>
    <string>MacOS</string>
    <key>ProductType</key>
    <string>iMac10,1</string>
    <key>ProductVersion</key>
    <string>10.7.3</string>
    <key>SerialNumber</key>
    <string>XXXXX</string>
    <key>UniqueDeviceID</key>
    <string>(hexadecimal string</string>
</dict>
</plist>

I had tried ~30 various combinations of Mac OS X as the deviceClass value with the albert.py from the pypush repo with no success before finally poking around the internet to find some sort of reference of a deviceClass value pre-macOS11 and came across the XML above.