AbstractBufferedFile does not appear to close an mmap cache

The result of mmap.mmap in MMapCache does not appear to get closed. That may be the reason for this error when repeatedly opening files stored on S3:

  File ".../python3.8/site-packages/fsspec/spec.py", line 1006, in open
    f = self._open(
  File ".../python3.8/site-packages/s3fs/core.py", line 489, in _open
    return S3File(
  File ".../python3.8/site-packages/s3fs/core.py", line 1659, in __init__
    super().__init__(
  File ".../python3.8/site-packages/fsspec/spec.py", line 1341, in __init__
    self.cache = caches[cache_type](
  File ".../python3.8/site-packages/fsspec/caching.py", line 58, in __init__
    self.cache = self._makefile()
  File ".../python3.8/site-packages/fsspec/caching.py", line 80, in _makefile
    return mmap.mmap(fd.fileno(), self.size)
OSError: [Errno 12] Cannot allocate memory

I believe that something like this in AbstractBufferedFile.close would fix it:

if self.mode == "rb":
    if isinstance(self.cache, MMapCache) and self.cache.cache:
        self.cache.cache.close()
    self.cache = None

A better solution may be to give all cache classes a close method that would be called at the indicated place.

Start in fsspec/spec.py at AbstractBufferedFile.close and inspect MMapCache in fsspec/caching.py, following how the cache is created and released. Reproduce repeated S3 opens if possible, then add coverage for closing an mmap-backed cache and verify resources are released without breaking other cache types.