add #PB_OS_MacOSX_10_15 for Catalina

[nsstudios]

Hi,

It would be nice if a constant for MacOS 10.15 (Catalina) could be added.

Thanks.

[hoerbie]

+1

And as we are half way between release of 10.15 and 10.16, this should precautionary be added too, if possible.

[kenmo]

+1

For PB releases, it should be on the release checklist to add any new OS versions which have come out.