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)

pyproject.toml

@@ -6,7 +6,6 @@ readme = "README.md"
 requires-python = ">=3.9"
 dependencies = [
     "deltas>=0.7.0",
-    "mediawiki-utilities>=0.4.18",
     "more-itertools>=10.7.0",
     "mwparserfromhell>=0.6.0",
     "mwpersistence>=0.2.4",
@@ -18,6 +17,17 @@ dependencies = [
     "yamlconf>=0.2.6",
 ]
 
+# Two features carry dependencies that are awkward enough to install that they
+# are kept out of the base install: -p legacy needs mediawiki-utilities, a 2015
+# package that also pulls in a MySQL client, and --diff and -p wikidiff2 need
+# pywikidiff2, which compiles a C++ extension. Everything else, including the
+# default -p sequence, works with neither.
+[project.optional-dependencies]
+legacy = ["mediawiki-utilities>=0.4.18"]
+# A wikidiff2 extra belongs here too, but pywikidiff2 is not yet published to
+# PyPI, so an extra naming it could not resolve. Until it is, wikiq reports the
+# git install command when --diff or -p wikidiff2 is used without it.
+
 [project.scripts]
 wikiq = "wikiq:main"