Please v12 released

[Peter Ebden]

This one has been incubating for a little while, but Please 12 is now done!

You can grab binaries from the release page or via Please's self-update.

The headline feature is a complete overhaul of the BUILD file parser; there is now no cgo / cffi involved and no requirement for Python to be installed on a machine during initial bootstrap.

The still-unnamed BUILD language is now a much tighter subset of Python, which will hopefully not present too many issues unless one is doing particularly fiddly things in BUILD files. See the documentation for specific information about what's supported.

Other features include:

• Go rules now filter their sources via build tags (including implicit ones derived from filenames) as go build would. This can be suppressed by setting filter_srcs = False if the old behaviour is preferred.
• The default Python interpreter is now python3. It can be set to python in the .plzconfig if one prefers the old behaviour.
• The GoVersion config attribute has been removed, and with it support for versions < 1.5.
  
• The BUILD file linter (accessed via plz tool lint) has been removed. The warnings that it reported are essentially all now prohibited by the new parser.
• The RPC cache now uses gzip compression; the server will need to be updated to the latest version to be able to communicate with clients.
  

Plus a bunch of other smaller features & bugfixes (and maybe some new bugs to go with them...)

Thanks to the various contributors to this release!