(pytest) Run one test against several environmental variable configurations

Is it possible to run tests with pytest such that some specific tests are run against different settings of an environmental variable?

My usecase is as follows: some of my tests use a machine learning library called Keras, but the backend of Keras can either be "Theano", "Tensorflow" or "CNTK". To change the backend I have to set the environmental variable "KERAS_BACKEND" accordingly before importing Keras. As a result, I'd like to run the test against those three different settings of the env variable.