ci: update integ dependencies

[jmayclin]

Update CI dependencies used during integration testing

Description of changes:

pytest 5.3 was released 3 years ago, it is time to update dependencies.

This upgrade gives us access to the following features.

pytest -n auto

This would automatically set the number of test runners equal to the number of workers on the instance. However this can't be used because port collisions happen for not-well-understood reasons: #2127

pytest --durations=100

This prints out the 100 slowest tests in a given run. This give us some idea about how long the individual tests are taking to run, and helps us make sure that there aren't any outliers really increasing the runtime.

Call-outs:

271: 1.27s call     test_version_negotiation.py::test_s2nc_tls13_negotiates_tls12[S2N-OpenSSL-TLS1.2-RSA_4096_SHA512-P-521-DHE-RSA-AES128-SHA256]
--
271: 1.23s call     
test_version_negotiation.py::test_s2nc_tls13_negotiates_tls12[S2N-OpenSSL-TLS1.2-RSA_2048_SHA384-P-384-DHE-RSA-AES256-GCM-SHA384]

Tests were slower than I expected them to be, taking more than a second for each combinatorially exploded fixture configuration.

Testing:

All CI should pass

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[dougch]

The CMake version of the pytest command should probably match. #4030 is another attempt at tracking timing by using junit reports, again, only on newer versions of pytest.

[jmayclin]

When not using Nix, the CMake version of the command still shells out to Tox and will use the same command.

In the Nix usecase, the testing commands have already diverged (e.g. Nix uses reruns set to 0 and doesn't use Tox at all).

My motivation for adding the durations command is that the integv2 job is currently the longest running mandatory CI job that we have and I want to make sure we aren't accidentally doing something silly, but that motivation doesn't quite apply to the Nix jobs.

[dougch]

We can do the nix update separately- this lays the groundwork for them to match.