pycompat: adding Linux detection and fixing Mac Python 3 recommends detecting OSs with the prefix of the platform, but we were comparing the full string for macOS. We also didn't have Linux detection, which is convenient for extensions to use (rather than have some OSs detected by hg and some by the extension). Reference: https://docs.python.org/3/library/sys.html#sys.platform Differential Revision: https://phab.mercurial-scm.org/D5227