Update documentation for version 4.0.0

[schwa]

Some of the following items apply more to the announcement than to the actual documentation.

• ftputil is now Python-3-only (Python 3.6+). Lots of old code for Python 2 support removed.
• Describe enhancement: methods which take a file name now also take a path-like object (#119)
• Time shift is now between the time used in server listings and UTC (previously between time in server listing and client local time), #134. FTPHost.(l)stat and FTPHost.path.getmtime use UTC time if time shift is set correctly. Add a documentation section to describe the related problems in detail.
• DIR option -a is no longer used by default (#110).
• Bug fixes (#131, #136)

[schwa]

Done as of [7d2ec0a6382d].