Update test with new invalid email error message

[gene1wood]

• Add tox environment serial to allow running tests serially
  • This removes the pytest addopts so we can have different ones in each tox environment.
  • This was needed as the -n=auto argument is a pytest-xdist argument that only works if pytest-xdist is used.
• Fix intermittent duo failure
  • This detects if the duo OTP entered returns an error then wait one second and tries a second time.
• Rename empty variables.json to an example file
  • Create a gitignore to prevent committing a variables.json with a real payload
• Update test to expect new error message format for invalid email address format

Fixes #27