* do not only return the 10 items from the first page, but loop over at most
`max_pages` pages
* use https://usa.anarchistlibraries.net/ as fallback when the main site is
not available
* drop formats TXT, TEX, MUSE from search results as they cannot be displayed
in calibre
* on python3 use quote_plus instead of quote
* obtain the version number in the user agent string from module
TheAnarchistLibraryStore
