Move ssh and local connection plugins from using raw select to selectors

At the moment, this change will use EPoll on Linux, KQueue on *BSDs,
etc, so it should alleviate problems with too many open file
descriptors.

* Bundle a copy of selectors2 so that we have the selectors API everywhere.
* Add licensing information to selectors2 file so it's clear what the
  licensing terms and conditions are.
* Exclude the bundled copy of selectors2 from our boilerplate code-smell test
* Rewrite ssh_run tests to attempt to work around problem with mocking
  select on shippable

Fixes ansible#14143

lib/ansible/compat/selectors/__init__.py

@@ -0,0 +1,47 @@
+# (c) 2014, 2017 Toshio Kuratomi <[email protected]>
+#
+# This file is part of Ansible
+#
+# Ansible is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# Ansible is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with Ansible.  If not, see <http://www.gnu.org/licenses/>.
+
+# Make coding more python3-ish
+from __future__ import (absolute_import, division, print_function)
+__metaclass__ = type
+
+'''
+Compat selectors library.  Python-3.5 has this builtin.  The selectors2
+package exists on pypi to backport the functionality as far as python-2.6.
+'''
+# The following makes it easier for us to script updates of the bundled code
+_BUNDLED_METADATA = { "pypi_name": "selectors2", "version": "1.1.0" }
+
+import os.path
+import sys
+
+try:
+    # Python 3.4+
+    import selectors as _system_selectors
+except ImportError:
+    try:
+        # backport package installed in the system
+        import selectors2 as _system_selectors
+    except ImportError:
+        _system_selectors = None
+
+if _system_selectors:
+    selectors = _system_selectors
+else:
+    # Our bundled copy
+    from . import _selectors2 as selectors
+sys.modules['ansible.compat.selectors'] = selectors