Please v10.3.0

Version 10.3.0 is done!

The headline feature in this release is being able to swap the Python test runner to be pytest instead of unittest (via the testrunner property under [python] in .plzconfig). Unittest tests have also been optimised a bit for faster rebuilding using some of the same mechanisms.

Other notable features include cgo_library rules accepting a pkg_config argument to determine appropriate compiler flags for dependencies, and the ability to opt individual go_library rules out of coverage instrumentation.

Thanks to contributors, and to the bugs & suggestions for these features.