switch from lockfile to filelock

[dimbleby]

I see that this project is rather inactive, but I guess I'm an optimist...

lockfile has long been deprecated, per https://github.com/ionrock/cachecontrol/issues/109 (2016!). https://github.com/ionrock/cachecontrol/pull/114 was an earlier try at getting cachecontrol onto something that was both active and better, it went with fasteners. These days filelock is getting about ten times as many downloads as fasteners - so I've used that.

Among the ways that this fixes actual bugs - ie is not just change for its own sake - is that the mechanisms used by filelock will automatically release the lock if the process exits early eg by Ctrl-C or bug or whatever.