Fix UnicodeEncodeError in default log file handler (#385)

[brantfaircloth]

Summary

• setup_logging() in phyluce/log.py creates a FileHandler without encoding="utf8" when --log-path is not given, unlike the --log-path branch which does specify it.
• phyluce_align_get_only_loci_with_min_taxa logs a message containing ≥, which raised UnicodeEncodeError on systems whose default file encoding is ASCII (as reported in Error in 'phyluce_align_get_only_loci_with_min_taxa'? #385).
• Adds encoding="utf8" to the default FileHandler so it matches the other branch.

Fixes #385

Test plan

• Run phyluce_align_get_only_loci_with_min_taxa without --log-path on a system with a non-UTF-8 default locale and confirm no UnicodeEncodeError is raised.