feat: Only activate pocketbook extractor optionally

Since we make the dependencies for pocketbook html extraction optional
as an extra, this commit ensures the extractor (and cli option) only
gets loaded when they exist.

papis_extract/extractors/__init__.py

@@ -1,8 +1,19 @@
+import papis.logging
+
 from papis_extract.extraction import Extractor
 from papis_extract.extractors import pdf
 from papis_extract.extractors.pocketbook import PocketBookExtractor
 
+logger = papis.logging.get_logger(__name__)
+
 all_extractors: dict[str, Extractor] = {
     "pdf": pdf.PdfExtractor(),
-    "pocketbook": PocketBookExtractor(),
 }
+
+try:
+    import bs4
+    import magic
+
+    all_extractors["pocketbook"] = PocketBookExtractor()
+except ImportError:
+    logger.debug("pocketbook extractor not activated.")