KeyAlreadyPresent exception does not strip whitespace from key

[ThatOtherAndrew]

Issue

If you have a TOML file containing something like the following:

my_key = 5
my_key = 10

Then the exception message will show the following:

tomlkit.exceptions.KeyAlreadyPresent: Key "my_key " already exists.

If you pad the equals sign with even more spaces, then the additional whitespace also shows up in the exception message. It's a minor detail, but it's been bugging me slightly.

Proposed solution

Substitute L197 in tomlkit.exceptions with the following:

message = f'Key "{key.rstrip()}" already exists.'

Additional notes

I'm not sure if this affects any other exceptions, but if it does, it may be worth using .rstrip() somewhere else. I'm not too familiar with the codebase, so I'm avoiding making a pull request as of now.