feat: add pkg extras for postgres, mysql; update install doc

no longer requires psycopg2 by default; use postgres extra

pyproject.toml

@@ -32,11 +32,12 @@ classifiers = [
 license = {text = "GNU General Public License v3+"}
 requires-python = ">= 3.8"
 dependencies = [
-        "psycopg2",
         "WuttaWeb>=0.20.5",
 ]
 
 [project.optional-dependencies]
+postgres = ["psycopg2"]
+mysql = ["mysql-connector-python"]
 docs = ["Sphinx", "furo", "sphinxcontrib-programoutput", "enum-tools[sphinx]"]
 tests = ["pytest-cov", "tox"]