Fix 2to3 basestring -> str
Fix 2to3 basestring -> str

lib2to3 changes basestring to str. For python3 this is excludes
bytestrings. Change to bytes to fix python3 and also allow str to
avoid breaking the API.

avalentino/PyTables,FrancescAlted/PyTables,PyTables/PyTables,PyTables/PyTables,FrancescAlted/PyTables,PyTables/PyTables,FrancescAlted/PyTables,avalentino/PyTables,avalentino/PyTables