Fix unicode/bytes handling in Tahoe config code

Fixes #225 (closed)

Note the target of this PR is #229 . Along with #231 this should make #229 green and then it can be merged into master to add Tahoe-LAFS 1.16.x testing to CI. For this branch alone many tests still fail against Tahoe-LAFS 1.16 but at least _zkapauthorizer.tests.test_plugin.ClientPluginTests.test_mismatched_ristretto_issuer passes now.