docs: Use 'req@url' syntax to install from remote VCS

[matthewfeickert]

• Use 'req@url' syntax when using pip to install from a remote git repository over 'url#egg=req' to avoid the use of #egg= fragments with a non-PEP 508 name. This will be required in pip v25.0+.
  • c.f. pypa/pip#11617 for more details.

[codecov[bot]]

Codecov Report

Base: 91.66% // Head: 91.66% // No change to project coverage :thumbsup:

Coverage data is based on head (85057ff) compared to base (60800a0). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #50 +/- ## ======================================= Coverage 91.66% 91.66% ======================================= Files 3 3 Lines 12 12 ======================================= Hits 11 11 Misses 1 1 ``` | Flag | Coverage Δ | | |---|---|---| | unittests-3.10 | `?` | | | unittests-3.7 | `?` | | | unittests-3.8 | `91.66% <ø> (ø)` | | | unittests-3.9 | `91.66% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pyhf#carryforward-flags-in-the-pull-request-comment) to find out more. 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=pyhf). 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=pyhf)

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