Add py as dependency (#555)

This dependency was implicitely set through pytest. With [1] the
dependency was removed from pytest, so pytest-html fails with:

ModuleNotFoundError: No module named 'py.xml'; 'py' is not a package

Specify it explicitely as dependency to fix the issue.

[1] pytest-dev/pytest@19dda7c

Signed-off-by: Erik Bloß <erik.bloss@smartmicro.de>

Signed-off-by: Erik Bloß <erik.bloss@smartmicro.de>

setup.py

@@ -13,7 +13,7 @@
     package_data={"pytest_html": ["resources/*"]},
     entry_points={"pytest11": ["html = pytest_html.plugin"]},
     setup_requires=["setuptools_scm"],
-    install_requires=["pytest>=5.0,!=6.0.0", "pytest-metadata"],
+    install_requires=["py>=1.8.2", "pytest>=5.0,!=6.0.0", "pytest-metadata"],
     license="Mozilla Public License 2.0 (MPL 2.0)",
     keywords="py.test pytest html report",
     python_requires=">=3.6",