Error when processing file mode string

In the memory filesystem there is an error if you try to open it with r+b mode saying this mode is not supported.

However this mode is apparently supported (https://github.com/fsspec/filesystem_spec/blob/master/fsspec/implementations/memory.py#L178).

What I think is happening is that the mode=r+b(or mode=rb+) argument is being processed and transformed internally to r+b and then the comparison to rb+ fails.

(I assume that it should work for any order for the characters in the mode string: rb+ or r+b).

Reproducer:

import fsspec
uri = "memory://path/file"
open_file = fsspec.open(uri, mode="rb+")  # r+b doesn't work either
assert open_file.mode == "rb+"
file = open_file.open() # raises ValueError

Start with fsspec/implementations/memory.py around line 178 and run the reproducer from the issue for both rb+ and r+b modes. Trace where the mode is normalized before the support check; done means both equivalent mode strings open successfully without the unsupported-mode error.