chore: Apply minor code quality revisions

[akshgpt7]

Description

I ran DeepSource analysis on my fork of the repo, and found some interesting code quality issues that can improve the general code quality here.
This PR fixes a few of the issues detected.

Summary of changes:

• Refactor unnecessary use of elif/else after return.
• Simplify if statements to use bool().
• Refactor iterating over dictionary instead of using dict.keys().
• Refactor unnecessary dict() call.

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Apply minor code changes
   - Use raise error as default behavior to avoid if-else evaluation
   - Use evaluation of bool() over trivial if statements
   - Drop use of keys on dict iteration

[codecov]

Codecov Report

Merging #1325 (a804557) into master (f1d1252) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1325   +/-   ##
=======================================
  Coverage   97.50%   97.50%           
=======================================
  Files          63       63           
  Lines        3760     3760           
  Branches      538      538           
=======================================
  Hits         3666     3666           
  Misses         55       55           
  Partials       39       39           

Flag	Coverage Δ
unittests	97.50% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/pyhf/modifiers/__init__.py	86.44% <100.00%> (ø)
src/pyhf/pdf.py	96.40% <100.00%> (ø)
src/pyhf/readxml.py	94.33% <100.00%> (ø)

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[akshgpt7]

@matthewfeickert The test seems to be failing for macOS when it tries to install tensorflow. Can you help me know if this is a fixable error?

[matthewfeickert]

Thanks for your PR @akshgpt7! We'll take a look at this later today and get it reviewed, which should be easy enough. We're probably not interested in picking up additional config files or dependencies at the moment though, so right off the bat can we ask you to remove .deepsource.toml?

The test seems to be failing for macOS ...

The VMs for GHA sometimes can be finicky for Mac. I restarted the CI.

[akshgpt7]

@matthewfeickert removed the .deepsource.toml file as requested.

[matthewfeickert]

Thanks for the PR. Though this PR doesn't really deal with code quality issues, but rather is just applying changes from the scan blindly. That's fine, but some of them don't really justify the noise they would add.

If you take out the ones that I've flagged I think we'd accept this.

[matthewfeickert]

Thanks. LGTM 👍