erp5: add yapf egg

https://github.com/google/yapf
A formatter for Python files

The initial idea is to use it with "format document" feature of monaco
editor.

stack/erp5/buildout.cfg

@@ -518,6 +518,7 @@ eggs = ${neoppod:eggs}
 # Needed for checking ZODB Components source code
   pylint
   jedi
+  yapf
   pytracemalloc
   xlrd
 
@@ -832,3 +833,5 @@ cookies = 2.2.1
 
 jedi = 0.15.1
 parso = 0.5.1
+
+yapf = 0.28.0