check_GNU_style_lib.py: Suggest to install all missing pip3 packages at once

Instead of:
...
$ ./contrib/check_GNU_style.py
termcolor module is missing (run: pip3 install termcolor)
$ pip3 install termcolor
$ ./contrib/check_GNU_style.py
unidiff module is missing (run: pip3 install unidiff)
$ pip3 install unidiff
$
...

Do:
...
$ ./contrib/check_GNU_style.py
termcolor and unidiff modules are missing (run: pip3 install termcolor unidiff)
$ pip3 install termcolor unidiff
$
...

2017-05-29  Tom de Vries  <tom@codesourcery.com>

	* check_GNU_style_lib.py: Use import_pip3 to import pip3 packages.
	(import_pip3): New function.

From-SVN: r248554