docs: document feature analysis for sections
acpt: add doc-access-sections.feature

New version of flake8 caught some new PEP8 bits in conf.py
sect: add Document.sections

and DocumentPart.sections
sect: add Sections.__len__()
sect: add Sections.__iter__()
sect: add Sections.__getitem__()
acpt: add act-section-props.feature
sect: add Section.start_type getter

Added InvalidXmlError to docx.exceptions, caused a circular import
error when trying to get it into enum from xmlchemy.
acpt: add scenarios for Section.start_type setter
sect: add Section.start_type.setter

Add key check to XmlEnumeration.to_xml() that raises ValueError on
value not a member of enumeration.
acpt: add scenario for Section.page_width
sect: add Section.page_width getter
sect: add Section.page_width getter
acpt: add scenario for Section page size setters
sect: add Section.page_width setter
sect: add Section.page_height setter
acpt: add scenarios for Section.orientation getter

Also, add WD_ORIENTATION enumeration.
sect: add Section.orientation getter
acpt: add scenarios for Section.orientation setter
sect: add Section.orientation setter