gnulib-tool.py: Don't leave temporary directories on exit.

gnulib-tool.py: Don't leave temporary directories on exit.

* pygnulib/main.py (main_with_exception_handling): Use
tempfile.TemporaryDirectory as a context manager so it is removed before
the program exits.
(main): Expect a temporary directory to be passed as an argument.
* pygnulib/GLConfig.py (GLConfig.__init__): Accept an optional temporary
directory parameter instead of creating one.
* pygnulib/GLImport.py (GLImport.__init__): Don't remove the cache's
temporary directory since it doesn't create one anymore.
(GLImport.execute): Don't remove the temporary directory explicitly. It
is handled by the usage of a context manager.
* pygnulib/GLTestDir.py (GLTestDir.execute, GLMegaTestDir.execute):
Likewise.