requirements: bump pytest-html to 2.1.x version
Version 2.0.x does not support hook - pytest_html_report_title.
Otherwise it fails with:
INTERNALERROR> pluggy.manager.PluginValidationError: unknown hook 'pytest_html_report_title' in plugin <module 'conftest' from '/foo/nsfarm/conftest.py'>
Reference (it is not included in commit description): https://github.com/pytest-dev/pytest-html/issues/279