contrib/linux-wheel-centos5-blacklist

selectors2: do not use platform.system()

`platform.system()` may have a side effect spawning a shell executing
`uname -p`, which may print a warning when the current directory is removed:

  shell-init: error retrieving current directory: getcwd: cannot access parent directories: No such file or directory

This patch changes selectors2 to test the `sys.platform` string, which is a
much safer way to detect Jython.

Jython's `sys.platform` looks like this:

  Jython 2.7.1 (default:0df7adb1b397, Jun 30 2017, 19:02:43)
  [OpenJDK 64-Bit Server VM (Oracle Corporation)] on java1.8.0_144
  Type "help", "copyright", "credits" or "license" for more information.
  >>> import sys
  >>> sys.platform
  'java1.8.0_144 ( ==linux2 for targets )'

Differential Revision: https://phab.mercurial-scm.org/D1018

test-convert-git.t
test-subrepo-git.t
test-patchbomb-tls.t