Add -Wno-error=declaration-after-statement to extra_compile_args

See http://bugs.python.org/issue21121

Fixes #639.
Add comment describing the workaround and linking to the Python 3 issue
Merge pull request #640 from jairideout/issue-639

Add -Wno-error=declaration-after-statement to extra_compile_args
ENH: io handles edge cases better