A recent experience with Travis-CI[1] showed that some tests were not
running with the intended Python version.  Let's add the Python
version the configure output, which serves as a general debugging
aid that the intended Python version was used[2][3].

Additionally, the recently introduced "check-venv" target, used by the
"check-acceptance" target, verifies if the configured Python interpreter
is Python 3, and does so on the Makefile itself.  Since the Python version
is being captured on configure, let's avoid rerunning Python on every
make invocation.

Finally, a small cosmetic fix to the "make check-help" output.

[1] https://travis-ci.org/clebergnu/qemu/jobs/452033247#L983
[2] https://travis-ci.org/clebergnu/qemu/jobs/452663112#L960
[3] https://travis-ci.org/clebergnu/qemu/jobs/452663113#L956

Cleber Rosa (4):
  configure: keep track of Python version
  check-venv: use recorded Python version
  Travis CI: make specified Python versions usable on jobs
  check-help: visual and content improvements

 .travis.yml            |  4 +++-
 configure              |  6 +++++-
 tests/Makefile.include | 11 ++++++-----
 3 files changed, 14 insertions(+), 7 deletions(-)

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2.19.1