String annotations [PEP 563] (#4390)

* Document `from __future__ import annotations`
* Provide plumbing and tests for `from __future__ import annotations`
* Implement unparsing the AST back to string form

This is required for PEP 563 and as such only implements a part of the
unparsing process that covers expressions.
diff --git a/PCbuild/pythoncore.vcxproj b/PCbuild/pythoncore.vcxproj
index fbcd051..f309455 100644
--- a/PCbuild/pythoncore.vcxproj
+++ b/PCbuild/pythoncore.vcxproj
@@ -358,6 +358,7 @@
     <ClCompile Include="..\Python\asdl.c" />
     <ClCompile Include="..\Python\ast.c" />
     <ClCompile Include="..\Python\ast_opt.c" />
+    <ClCompile Include="..\Python\ast_unparse.c" />
     <ClCompile Include="..\Python\bltinmodule.c" />
     <ClCompile Include="..\Python\bootstrap_hash.c" />
     <ClCompile Include="..\Python\ceval.c" />