requirements: bump pytest-html to 2.1.x version

Version 2.0.x does not support hook - pytest_html_report_title.
It fails with:

INTERNALERROR> pluggy.manager.PluginValidationError: unknown hook 'pytest_html_report_title' in plugin <module 'conftest' from '/foo/nsfarm/conftest.py'>

requirements.txt

 pytest>=5.0
 pytest-lazy-fixture>=0.5
-pytest-html>=2.0
+pytest-html>=2.1.0
 pexpect
 pyserial
 pylxd