[cleanup] Make sure that files are closed after being opened.

[mapellidario]

description

During the python3 migration, we notices a couple of occurrences of files that are opened, read but not closed.

A couple of examples are

• https://github.com/dmwm/CRABServer/blob/0b1c2ec6bbe34f81460403bc7b604a5085c3f4ca/scripts/cmscp.py#L913
• https://github.com/dmwm/CRABServer/blob/0b1c2ec6bbe34f81460403bc7b604a5085c3f4ca/src/python/CMSGroupMapper.py#L78

desired solution

We should make sure that every file is opened within a context manager, so that we are sure it is going to be properly closed, eventually.

[belforte]

of course cmscp.py is a script which is executed once in per job in the WorkerNode. So everything is closed when it exits. ANd besides this, cmscp.py could surely use a good cleanup and refactoring. But we need to be very careful with persistent processes which may result in file descriptor leaks.