Add a test to ensure that "a = (1, 2)" is parsed and printed correctly.

Due to a bug in ast-types, this expression is re-printed as `a = 1, 2`.

https://github.com/benjamn/ast-types/pull/32
Merge pull request #75 from eventualbuddha/fix-sequence-as-assignment-rhs

Add a test to ensure that "a = (1, 2)" is parsed and printed correctly.