Ipykernel

asyncio compatibility.

- Add github actions, bail on asyncio patch for tornado 6.1.
[564](https://github.com/ipython/ipykernel/pull/564)
- Start testing on Python 3.9. [551](https://github.com/ipython/ipykernel/pull/551)
- Fix stack levels for ipykernel's deprecation warnings and stop
using some deprecated APIs. [547](https://github.com/ipython/ipykernel/pull/547)
- Add env parameter to kernel installation [541](https://github.com/ipython/ipykernel/pull/541)
- Fix stop_on_error_timeout blocking other messages in queue.
[539](https://github.com/ipython/ipykernel/pull/539)
- Remove most of the python 2 compat code. [537](https://github.com/ipython/ipykernel/pull/537)
- Remove u-prefix from strings. [538](https://github.com/ipython/ipykernel/pull/538)

- Only run Qt eventloop in the shell stream. [531](https://github.com/ipython/ipykernel/pull/531)

- Fix QSocketNotifier in the Qt event loop not being disabled for the
control channel. [525](https://github.com/ipython/ipykernel/pull/525)

- Restore timer based event loop as a Windows-compatible fallback.
[523](https://github.com/ipython/ipykernel/pull/523)

- Fix 520: run post_execute and post_run_cell on async cells
[521](https://github.com/ipython/ipykernel/pull/521)
- Fix exception causes in zmqshell.py [516](https://github.com/ipython/ipykernel/pull/516)
- Make pdb on Windows interruptible [490](https://github.com/ipython/ipykernel/pull/490)