scripts/jobserver-exec: move the code to a class

scripts/jobserver-exec: move the code to a class

Convert the code inside jobserver-exec to a class and
properly document it.

Using a class allows reusing the jobserver logic on other
scripts.

While the main code remains unchanged, being compatible with
Python 2.6 and 3.0+, its coding style now follows a more
modern standard, having tabs replaced by a 4-spaces
indent, passing autopep8, black and pylint.

The code allows using a pythonic way to enter/exit a python
code, e.g. it now supports:

	with JobserverExec() as jobserver:
	    jobserver.run(sys.argv[1:])

With the new code, the __exit__() function should ensure
that the jobserver slot will be closed at the end, even if
something bad happens somewhere.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Message-ID: <4749921b75d4e0bd85a25d4d94aa2c940fad084e.1758196090.git.mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>