pyhf v0.7.4 released

[pyhf announcements]

Dear pyhf users,

We're happy to announce that pyhf v0.7.4 is out now on PyPI and Conda-forge! This is a patch release with fixes contributed by Daniel Werner and Jonas Rembser. There are no API changes and no changes in dependency lower bounds, so we encourage you to update to v0.7.4

```
python -m pip install --upgrade pyhf
```

Below we list the most important points from the release notes:

* Skip callbacks with dead weakrefs while iterating over callbacks in pyhf events, like pyhf.set_backend, to avoid the possibility of accessing dead weakrefs before they could be garbage collected.

The fixed bug was subtle and occurred nondeterministically when the pyhf.tensorlib was changed repeatedly causing dead weakrefs to be accessed before Python's garbage collection could remove them. Most users should be unaffected.

Thank you for using pyhf and for continuing to provide us feedback on both new feature requests as well as sharing your bug reports and user experiences through the recommended channels.

Best,
The pyhf dev team (Matthew, Lukas, and Giordon)