It fails when use all severity types in "--allure-severities" option

[sseliverstov]

I'm submitting a ...

• bug report
• feature request
• support request => Please do not submit support request here, see note at the top of this template.

What is the current behavior?

If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem

What is the expected behavior?

What is the motivation / use case for changing the behavior?

Please tell us about your environment:

• Allure version: 2.1.0
• Test framework: pytest@3.0
• Allure adaptor: allure-pytest@2.0.0b1

Other information

[delatrie]

Reproduction:

Given pytest and allure-pytest installed, if we run:

pytest --allure-severities=blocker,critical,normal,minor,trivial --alluredir=./.allure-results

Then the following error occurs:

pytest: error: argument --allure-severities: Illegal severity values: , only [normal, critical, trivial, blocker, minor] are allowed

Notes for future contribution:

Use <= instead of < here (if all severity levels are provided, the set of the allowed values is equal to the set of the provided values):

allure-python/allure-pytest/src/plugin.py

Line 47 in d667b5e

if not atoms < legal_values:

See also #260