MLBEDSW-7579: Fix test_build.py test issues

* Fix import order in test_build.py
* Fix setup_tools_scm dependency version. Previously the version was
  restricted to < 6, creating a version restriction on Setuptools
  library too.
  Because an older version of Setuptools was used, running
  test_build.py::test_build_correct_readme_links would generate a
  UNKNOWN.egg-info directory in the src directory instead of a
  ethos_u_vela.egg-info directory.

Change-Id: I113ca25b23b39d43fa288e6eda16377f4f5b4143
Signed-off-by: Raul Farkas <raul.farkas@arm.com>