Add linter for detecting open files that are never closed

[mschwager]

Most file IO should be using with statements to automatically close files, but there still may be instances of manual open and close calls (or lack thereof). See Reading and Writing Files.

There are lots of ways a file can be opened:

• open
• os.open
• io.open (alias for builtin open)
• tempfile.TemporaryFile|NamedTemporaryFile|SpooledTemporaryFile
• tarfile.open
• ZipFile.open
• Others?

My first idea for an implementation is tracking variable instantiation of the above methods, then checking for a lack of a close call in the same scope.

Also would be good to check for a lack of closing a connection after opening one (e.g. in SQLAlchemy). There's probably lots of opportunity for this in DB connection libraries.