Coverage.py predates sets as a built-in data structure, so the file data
collection has long been dicts with None as the values.  Sets are
available to us now (since Python 2.4 in 2004, which coverage.py dropped
support for in 2014!), we use sets.

"file_data" used to be borrowed from data, but that was confusing. Now it's
owned.  We used to have a struct member which was a copy of the current stack
entry.  That just made it harder to reason about reference counting.  Now we
have a pointer to the entry on the stack.