const: migrate CONTENT_TYPE to opc_constants.py
const: migrate RELATIONSHIP_TYPE to opc_constants
oxml: refactor namespace prefixed tag handling
test: refactor to use assertEqualLineByLine
iss36: remove indiscriminate deannotation
opc: remove pretty-print of saved XML
test: add [pytest] section to tox.ini
refactor: rename file to file_ in open() methods

'file' is a reserved word. Appending underscore to reserved words to
form a variable name is some sort of convention.