[Feature Request] Make percy finalize only once

[byronz]

we are currently limited by the Percy Python API in a way that its lifecycle is bound with a selenium webdriver. and there is no easy way to finalize that with a simple direct API call from Percy itself.

another problem we are having in dash repo is that due to the limitation of circleci workflow, a failure of test run will skip the proper percy finalize job.

Proposed solution:
if we can find out a way to properly finalize the percy runner with Python, then we can define a percy fixture with scope='session', and doing the finalize with the pytest lifecycle.

[byronz]

solved by plotly/dashR#138