Eliminate white space around ristretto key when reading key file

[hacklschorsch]

It is easy to erroneously add extra white space to a text file. For example when writing with an editor like nano or when using echo without the -n argument.

Handing that white space over to python-challenge-bypass-ristretto's decode_base64() method will make it fail in a rather opaque way, see LeastAuthority/python-challenge-bypass-ristretto#37.

This eliminates white space including newlines around the key expected to be in that file.

[exarkun]

Thanks. Do you feel up to writing a unit test, too?

[hacklschorsch]

I tried for one hour to run the existing tests, then gave up.

[exarkun]

Yea, fair :( I also started looking at the state of the test suite and it's not great. I'll loop back here after it has been revivified...

[hacklschorsch]

I could also make the point that Unit testing here might not be the right thing:

• the SigningKey.decode_base64() unit is imported from another library and tested there. It specifies the parameter it expects, and it's our job to adhere to the spec
• the FilePath(...).getContent() unit is from the standard library and widely tested,
• as is the strip() unit.

It's the integration of all this we want to test.
Testing that integration makes good sense, and I also think it is a good location to spec our expected input.

I think it is worth while to add an integration test for this.

But honestly, trying to be very street smart I want to do something else.

[hacklschorsch]

That violates the principle that Unit tests should test one thing only. But OTOH, if some other component has a problem with a signing key like that, it should be exposed as well.

[exarkun]

In #198 I took a shot at the kind of "unit test" I think provides value here. I started w/ this PR for that branch so I'm gonna say that PR supersedes this one.