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.

Include/code.h

@@ -82,6 +82,7 @@ typedef struct {
 
 #define CO_FUTURE_BARRY_AS_BDFL  0x40000
 #define CO_FUTURE_GENERATOR_STOP  0x80000
+#define CO_FUTURE_ANNOTATIONS    0x100000
 
 /* This value is found in the co_cell2arg array when the associated cell
    variable does not correspond to an argument. */