lyrics: add tests to track which websites can be scraped by our algo and be
used as sources fot the google custom search engine.
fix flake8
Update lyrics_sources.py

add copyright header
change chars replacements done by slugify()

adapt slugify() to make it return strings that can be used
as yaml keys (no spaces, etc.)
adapt is_page_candidate() accordingly
is_page_candidate() handle more languages

Add translations in spanish and german for tokens to
ignore when comparing an url title with a song title.
Move lyrics listings from .py to .yaml file
remove _lyricstext.py
Merge branch 'KraYmer-lyrics-integration-sources'