Begin pydicom's own py2/3 compat module.  Invert boolean in_py3 to in_py2 comparisons.
Convert to pydicom's own py2/3 text_type.
Converts to pydicom's own py2/3 string_types
Converts all py2/3 compatibility code to pydicom's own compat module.
Merge branch 'dev' into modernize
Merge branch 'dev' of https://github.com/darcymason/pydicom into dev