feat: lock access to underlying cfitsio object

[beckermr]

This PR adds locks to prevent concurrent usage of the cfitsio object.

See https://py-free-threading.github.io/porting/#raising-errors-under-shared-concurrent-use.

There are a lot of potential issues here, but the main ones are that

• I have not structured the locks to ensure key operations are atomic.
• How the locks should be structured likely depends on the exact use case.

In other words, while I have ensured that no thread uses the same cfitsio file pointer at the same time, I have not ensured that a given thread can lock all of the python+C data structures long enough to do a fully consistent operation.

So for example, the lock is not held long enough for the thread to write image data via the C layer and also update the HDU list in the python layer.

So the locks as they are implemented now are formally correct, but don't really help the user avoid important race conditions.

[esheldon]

"each thread must open"

[beckermr]

I'm not sure if I am going to want to merge this one.

[beckermr]

I am going to close this pull request. I think locking should be the responsibility of the user since it is so use case dependent and likely needs to be done at the python level anyways.