Fix reading multiline secrets (e.g. files) with op.read()

[adnandaut]

• Resolves Using op.read() to pull the contents of a text file attached to an item only pulls the first line of the document #73

Side note:
I noticed that other places in the code use rstrip('\n').
But if an attached file acually ends with one or more line breaks, I think we want to leave them as they are and only remove the single trailing newline that the op cli tool adds as a convenience for interactive use in the shell.
For this, the method removesuffix, introduced in Python 3.9, is used instead of rstrip, which removes all trailing linebreaks.

Since I do not have enough context for the other usages of rstrip in the code, I can not say for certain, but I would think that they could also utilise removesuffix instead.