Eliminate white space around ristretto key when reading key file

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 https://github.com/LeastAuthority/python-challenge-bypass-ristretto/issues/37.

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

src/_zkapauthorizer/_plugin.py

@@ -145,7 +145,7 @@ class ZKAPAuthorizer(object):
         signing_key = SigningKey.decode_base64(
             FilePath(
                 kwargs.pop(u"ristretto-signing-key-path"),
-            ).getContent(),
+            ).getContent().strip(),
         )
         announcement = {
             u"ristretto-issuer-root-url": root_url,