Fix inconsistent file mode formatting and comparison (rb+ changed to r+b)

[lobis]

Triggered by #1425

The current processing of the file mode string causes the b character to always be on the end of the mode e.g. rb+ becomes r+b.

However in some instances (such as in the memory file system), the valid file modes listed are not compatible with this format, for instance rb+ is listed. This comparison will never take place given how the mode string is processed.

This PR replaces all instances of rb+ to r+b so that this works now.

[lobis]

@martindurant after starting this PR I realised that there are different places where the mode is specified (not everything goes through the OpenFile.enter) (This is why the new test fails). I could normalize the mode everywhere but it's a bit messy.

Do you agree on the need to normalize (sort) the file mode string? I think it's dangerous to do string comparisons inside the file system against some reference modes ("wb", "rb+", etc.) when these modes can be in different order (but meaning the same thing). Currently I think the only failing case is when using "+" but still I think it should be addressed (memory filesystem cannot be opened with "rb+" or "r+b" because of the reordering taking place).

[martindurant]

Maybe you can sprinkle set() or sorted() where exact equality is tried? In most cases, in is used instead.

[lobis]

At the end I chose not to modify how mode is processed: mode = self.mode.replace("t", "").replace("b", "") + "b".

Instead I replaced all instances of rb+ to r+b. Now it works fine.

I think it makes sense anyway that the file mode should always have b at the end since this is a python thing.

[lobis]

@martindurant this is now ready, please let me know if you any have feedback.

[martindurant]

python seems to agree