Although `yes` and `no` are recognized as booleans by the pyyaml parser,
the correct keywords are `true` and `false` (as highlighted by the newly
added `truthy` rule).
This commit replaces the use of `yes`/`no` by `true`/`false` and
advertise it in the docs, but also makes sure this change is
backward-compatible (so that `yes` and `no` still work).
This change fixes new errors detected by the last version of pycodestyle
(2.2.0), which is a dependency of flake8:
./tests/test_spec_examples.py:51:1: E305 expected 2 blank lines
after class or function definition, found 1
./tests/test_spec_examples.py:139:1: E305 expected 2 blank lines
after class or function definition, found 1
See pycodestyle changelog at 2.2.0 and
https://github.com/PyCQA/pycodestyle/pull/593.
