Fix for issue 528 - quoting string literals in list/dict/set

[charlesgwaldman]

Fix string quoting of constant collection elements in n_const_list

Fixes #528.

Elements of folded constant collections (BUILD_CONST_LIST/SET/KEY_MAP, 5+ elements) were misquoted. Add a new format_const() helper: emits pattr on 3.x, repr's the raw value on 2.x.

Fixes two cases:

2.7 list/set string elements printed unquoted
3.6+ const dict values printed double-repr'd (the old "%r" % pattr re-repr'd an already-repr'd string).

3.x list/set output is unchanged.

Note that there's a mix of single and double quotes in the 3.x disassembly but I think this is purely cosmetic.

D = {239: 240, 168: 169, 54: 55, 128: 129, 'fred': "wilma"}

Test: test/simple_source/bug27+/05_const_collection.py (self-checking), with 2.7 and 3.6 fixtures covering the list/set and dict paths respectively.

[rocky]

Looks good - thanks!