File paths and column names containing non-ASCII characters are mishandled

[jpgill86]

axographio is unable to open file paths containing non-ASCII characters. Instead, a UnicodeEncodeError exception (Python 3) or an IOError exception (Python 2) is raised.

Some files that contain only ASCII characters in the path but contain non-ASCII column names cannot be opened and may raise a UnicodeDecodeError exception (Python 3). Others can be opened, but their column names are decoded incorrectly.

These problems have been present since axographio's initial release, but the explicit instruction to Cython for encoding strings in ASCII introduced in commit 77e0ed3 to provide Python 3 compatibility didn't help.

These files can be used for testing: test-non-ascii.zip