Pytest-retry

Fixed the following bugs:

- Unhandled exceptions during teardown cause runtest process to fail on a stash KeyError

New featues:

- Final reported test durations are now based on the latest test attempt by default.
- The old behavior, where all attempt durations were summed into the final duration, can still be enabled via the --cumulative-timings=1 flag or via the flaky marker timings arg
- Instead of attaching a full report to a test item as an attr (discouraged by Pytest documentation). The outcome of the test and the number of attempts are now safely saved to each item's stash. Both keys `success_key` and `attempt_key` can be imported and used to access these values in other hooks or fixtures.

Changelog:

- Test durations are now overwritten by the most recent attempt by default. This is to provide more consistent data to time-based splitting algorithms, such as CircleCI's.
- new --cumulative-timings flag (defaults to False). Can be used to revert to the previous behavior where the duration of each attempt is summed together in the final report
- Latest report instance is no longer attached to the item as an attribute. The solution encouraged by the Pytest documentation is to use the item's Stash instance. Two new keys are used to store the number of attempts for each item and whether it passed or failed.
- Both stash keys can be imported from the plugin to access the outcome data for other purposes.

Changelog:

- The latest original report is now attached to the test item for easy access

Initial release of pytest-retry