Update lxml requirement from <=4.9.3 to <=4.9.4 (#173)

Updates the requirements on [lxml](https://github.com/lxml/lxml) to permit the latest version.
- [Release notes](https://github.com/lxml/lxml/releases)
- [Changelog](https://github.com/lxml/lxml/blob/master/CHANGES.txt)
- [Commits](lxml/lxml@lxml-0.5.1...lxml-4.9.4)

---
updated-dependencies:
- dependency-name: lxml
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

setup.py

@@ -251,7 +251,7 @@ def path_to_src(source_path_list):
        package_dir = {"": src_dir},
        packages = ["PyPop", "PyPop.xslt"],
        package_data={"PyPop.xslt": data_file_paths},
-       install_requires = ["numpy <= 1.26.2", "lxml <= 4.9.3",
+       install_requires = ["numpy <= 1.26.2", "lxml <= 4.9.4",
                            "importlib-resources; python_version <= '3.8'", "importlib-metadata; python_version <= '3.8'"],
        extras_require={
            "test": ['pytest']