build: Update lower bound of pyhf to v0.7.0rc2

[matthewfeickert]

v0.7.0rc2 fixes a regression in staterror introduced after v0.6.3 and so is present in v0.7.0rc1.

[codecov[bot]]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage :thumbsup:

Coverage data is based on head (fd3c197) compared to base (84c7367). Patch has no changes to coverable lines.

:exclamation: Current head fd3c197 differs from pull request most recent head d2fa5f8. Consider uploading reports for the commit d2fa5f8 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #361 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 23 23 Lines 2010 2009 -1 Branches 327 326 -1 ========================================= - Hits 2010 2009 -1 ``` | [Impacted Files](https://codecov.io/gh/scikit-hep/cabinetry/pull/361?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scikit-hep) | Coverage Δ | | |---|---|---| | [src/cabinetry/workspace.py](https://codecov.io/gh/scikit-hep/cabinetry/pull/361/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scikit-hep#diff-c3JjL2NhYmluZXRyeS93b3Jrc3BhY2UucHk=) | `100.00% <0.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scikit-hep). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scikit-hep)

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