Bundle PyYAML code with the pipeline.

The PyYAML project is considered mature, with no changes in over a
year, but there are some minor issues that pose a problem for the
pypeline use of YAML: Most importantly, duplicate keys are silently
ignored.

For now, only the Python 2.x version of PyYAML is bundled.
Always use python implementation.
Fix deprecation warning.
Disallow duplicate keys.
Switch to using bundled version of PyYAML.
Fix parsing of float in scientific notation without decimals.