pycompat: define operating system constants As suggested by Ryan in D1019, it's cleaner if we use defined constants instead of `osname == 'nt'` everywhere. Differential Revision: https://phab.mercurial-scm.org/D1033

hgweb: do not import uuid immediately to avoid its side effect With hgdemandimport disabled (chg's case), `import uuid` has an immediate side effect calling `ctypes.util.find_library` trying to locate the `libuuid` library. This happens at `import` time before `dispatch.run()`. The call trace is like: File "hg/hg", line 54, in <module> from mercurial import ( File "hg/mercurial/dispatch.py", line 24, in <module> from . import ( File "hg/mercurial/commands.py", line 23, in <module> from . import ( File "hg/mercurial/help.py", line 33, in <module> from .hgweb import ( File "hg/mercurial/hgweb/__init__.py", line 20, in <module> from . import ( File "hg/mercurial/hgweb/hgweb_mod.py", line 14, in <module> from .common import ( File "hg/mercurial/hgweb/common.py", line 15, in <module> import uuid File "/usr/lib64/python2.7/uuid.py", line 404, in <module> lib = ctypes.CDLL(ctypes.util.find_library(libname)) The problem is, `ctypes.util.find_library` will execute `sh -c '/sbin/ldconfig -p 2>/dev/null'` on Python <= 2.7.12. The output of `sh` may pollute the terminal: shell-init: error retrieving current directory: getcwd: cannot access parent directories: No such file or directory This patch moves `import uuid` so its side-effect can only happen after the cwd check in `dispatch._getlocal`. Therefore the terminal won't be polluted by importing `uuid`. Differential Revision: https://phab.mercurial-scm.org/D1024

check-code: forbid platform.system() See the previous patches for the reason. Differential Revision: https://phab.mercurial-scm.org/D1021