opc: add pptx.opc sub-package
reorg: move opc_constants module to opc subpackage
reorg: move packaging.py module to opc subpackage
reorg: move rels.py module to opc subpackage
reorg: move test_packaging.py to tests/opc/
reorg: move test_rels.py to tests/opc/
reorg: remove tests/context.py
reorg: rename tests/testing.py to unitutil.py
reorg: rename tests/testdata.py to unitdata.py
reorg: move absjoin() from test/* to unitutil.py
reorg: move test_file_dir var to unitutil.py
reorg: clean up and standardize file headers
reorg: remove redundant _child() function defs
reorg: migrate to_unicode to util module
reorg: change os.path.join to absjoin in tests
reorg: Package.__relationships to single underscore
reorg: Package.__walkparts to single underscore
reorg: Part.__partname to single underscore
reorg: Part.__relationships to single underscore
reorg: Relationship.__source to single underscore