Px for Windows

• Implemented #65 - support for specifying PAC in INI
• Implemented #73 - force auth mechanism in INI
• Merged PR #85 - sanitize authorization header, shutdown socket, defer
  connection header during auth (#71), drop completely for CONNECT, forward
  error body, 2.7 fix
• Switched to Python 3.7, drop 3.4 support
• Merged PR #80 - fallback to DIRECT if no proxy found via auto or PAC discovery
• Added basic auth support (PR #82)
• Fixed issue where an empty domain was breaking password access
• Fixed #88, issue 2 in #71, PR #83 - use auth mechanism in order
• Added code to keep-alive during authentication - issue 3 in #71