make mediawiki-utilities an optional dependency

-p legacy is the only thing that uses mediawiki-utilities, and it exists
solely to reproduce numbers from research predating the mwxml and
mwtypes split. The package is a 2015 monolith that also contains
mw.database and mw.api, so it declares requests and pymysql
unconditionally: every wikiq install pulled a MySQL client and six other
packages to support one flag, building them from an sdist that has no
wheel.

Move it to a `legacy` extra. mw.lib.persistence imports nothing from
mw.database or mw.api, so nothing about -p legacy changes for people who
install the extra, and everyone else stops paying for it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
(cherry picked from commit 66dcc6d8cea620a619fc19121b5c27a085b11991)

src/wikiq/__init__.py

@@ -67,6 +67,22 @@ def require_pywikidiff2(feature: str):
     return pywikidiff2
 
 
+def require_mw_persistence():
+    """Import and return mw.lib.persistence, or exit explaining how to get it."""
+    try:
+        from mw.lib import persistence
+    except ImportError:
+        raise SystemExit(
+            "-p legacy requires mediawiki-utilities, which wikiq no longer "
+            "installs by default.\n"
+            "Install it with:\n"
+            "    pip install 'mediawiki-dump-tools[legacy]'\n"
+            "This method exists to reproduce results from older research "
+            "projects; -p sequence is the current equivalent."
+        )
+    return persistence
+
+
 def pyarrow_type_to_spark(pa_type):
     """Convert a PyArrow type to Spark JSON schema format."""
     if pa.types.is_int64(pa_type):
@@ -845,7 +861,7 @@ class WikiqParser:
                         wikidiff_matcher, revert_radius=PERSISTENCE_RADIUS
                     )
                 else:
-                    from mw.lib import persistence
+                    persistence = require_mw_persistence()
                     persist_state = persistence.State()
 
             # Pending persistence values waiting for window to fill
@@ -1298,6 +1314,8 @@ def main():
         require_pywikidiff2("--diff")
     if persist == PersistMethod.wikidiff2:
         require_pywikidiff2("-p wikidiff2")
+    elif persist == PersistMethod.legacy:
+        require_mw_persistence()
 
     if args.namespace_filter is not None:
         namespaces = args.namespace_filter