Change default smart connection to ssh on macOS and remove paramiko from requirements.txt (#54738)

* Remove default use of paramiko connection plugin on macOS
    This fix was originally to work around a bug that caused a kernel panic on macOS
    that has since been fixed.
* Remove paramiko from requirements.txt
* Move paramiko checking to common place
* Drop the warnings obfiscation code
* Update pip installation instructions to reflect upstream instructions
* Fix tests on CentOS 6 (Python 2.6) that now show Python deprecation warnings
* Add changelog fragment

lib/ansible/module_utils/compat/paramiko.py

@@ -0,0 +1,19 @@
+# -*- coding: utf-8 -*-
+# Copyright (c) 2019 Ansible Project
+# Simplified BSD License (see licenses/simplified_bsd.txt or https://opensource.org/licenses/BSD-2-Clause)
+
+from __future__ import absolute_import, division, print_function
+__metaclass__ = type
+
+PARAMIKO_IMPORT_ERR = None
+
+paramiko = None
+try:
+    import paramiko
+except ImportError:
+    try:
+        import ansible_paramiko as paramiko
+    except (ImportError, AttributeError) as err:  # paramiko and gssapi are incompatible and raise AttributeError not ImportError
+        PARAMIKO_IMPORT_ERR = err
+except AttributeError as err:  # paramiko and gssapi are incompatible and raise AttributeError not ImportError
+    PARAMIKO_IMPORT_ERR = err