MemoryFilesSstem missing valid modes for open()

[orenl]

import fsspec

fs = fsspec.filesystem('memory')

with fs.open('hello', 'w+b') as f:
    f.write(b'world')

results in:

$ python3 /tmp/filemode.py
Traceback (most recent call last):
  File "/tmp/try2.py", line 5, in <module>
    with fs.open('hello', 'w+b') as f:
         ^^^^^^^^^^^^^^^^^^^^^^^
  File "virtualenvs/app-tNTrAeIV-py3.12/lib/python3.12/site-packages/fsspec/spec.py", line 1338, in open
    f = self._open(
        ^^^^^^^^^^^
  File "virtualenvs/app-tNTrAeIV-py3.12/lib/python3.12/site-packages/fsspec/implementations/memory.py", line 211, in _open
    raise ValueError(f"unsupported file mode for {name}: {mode!r}")
ValueError: unsupported file mode for MemoryFileSystem: 'w+b'

The open() method should respect all valid modes, including "w+b", "a+b", "x+b".
Will follow up with a PR to fix this behavior.

Note also that the memory filesystem does not repsect "read-only"/"write-only" properties; However fixing this is more involved change, and may impact backward compatibilty for users that rely on such (broken) behavior, and nobody complained so far.