Broken SSL in MacOS CI

As of this morning, all test suites fail on MacOS github actions, e.g.

https://github.com/dask/distributed/runs/4534704460?check_suite_focus=true

```
______________ ERROR collecting distributed/comm/tests/test_ws.py ______________
ImportError while importing test module '/Users/runner/work/distributed/distributed/distributed/comm/tests/test_ws.py'.
../../../miniconda3/envs/dask-distributed/lib/python3.7/site-packages/cryptography/hazmat/bindings/openssl/binding.py:14: in
from cryptography.hazmat.bindings._openssl import ffi, lib
E ImportError: dlopen(/Users/runner/miniconda3/envs/dask-distributed/lib/python3.7/site-packages/cryptography/hazmat/bindings/_openssl.abi3.so, 2): Library not loaded: @rpath/libssl.1.1.dylib
E Referenced from: /Users/runner/miniconda3/envs/dask-distributed/lib/python3.7/site-packages/cryptography/hazmat/bindings/_openssl.abi3.so
E Reason: image not found
```

The cryptography package hasn't been updated recently.