Tom Coldrick authored Dec 20, 2018 and Tristan Van Berkom committed Jan 03, 2019

In _yaml.py and _frontend/complete.py we were getting pylint warnings
for using collections.Mapping and collections.Iterable, which are
abstract classes now provided from collections.abc. This patch just
uses the classes from the right place.

Use collections.abc for Mapping, Iterable

Closes #831

See merge request !1020

This seems like a better name for the directory, as it more closely
describes the purpose of its contents.

Split the "The MANIFEST.in and setup.py" section in two:
"Managing data files" and "Updating BuildStream's Python dependencies".

Briefly explain the layout of `requirements` directory and add
instructions to use the Makefile added in the last commit.