Upgrade pip before installing astropy in CI

Installation of astropy in CI fails after the release of astropy 4.0.2 It turned out that astropy is now relying on some newer features in the pip installer. Currently the pip installer used in CI is rather old (9.0.1).

This MR lets pip upgrade itself after the system install, after which astropy can be installed without problems. The disadvantage of the pip upgrade is that it installs a newer version parallel to the system install. This causes problems when invoking pip through the wrapper script, see https://github.com/pypa/pip/issues/5599 A reliable work-around is to call pip through python: python3 -m pip. This MR uses that work-around in the CI script.