Add return values to functions to fix mypy warnings

[gshank]

resolves #8389

Problem

Mypy warnings cluttering up mypy errors

Solution

Add return values to functions in:

• core/dbt/adapters/base/query_headers.py
• core/dbt/clients/jinja.py
• core/dbt/context/macro_resolver.py
• core/dbt/contracts/graph/unparsed.py
• core/dbt/graph/selector_methods.py
• core/dbt/parser/common.py
• core/dbt/semver.py
• core/dbt/task/run.py
• core/dbt/tests/util.py

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX

[codecov]

Codecov Report

Merging #8416 (7466200) into main (cb4bc2d) will decrease coverage by 0.06%.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #8416      +/-   ##
==========================================
- Coverage   86.36%   86.30%   -0.06%     
==========================================
  Files         174      174              
  Lines       25559    25559              
==========================================
- Hits        22074    22059      -15     
- Misses       3485     3500      +15     

Flag	Coverage Δ
integration	83.09% <100.00%> (-0.12%)	⬇️
unit	65.14% <96.15%> (ø)

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

Files Changed	Coverage Δ
core/dbt/adapters/base/query_headers.py	98.41% <100.00%> (ø)
core/dbt/clients/jinja.py	88.85% <100.00%> (ø)
core/dbt/context/macro_resolver.py	99.07% <100.00%> (ø)
core/dbt/contracts/graph/unparsed.py	93.10% <100.00%> (ø)
core/dbt/graph/selector_methods.py	91.48% <100.00%> (ø)
core/dbt/parser/common.py	92.38% <100.00%> (ø)
core/dbt/semver.py	90.45% <100.00%> (ø)
core/dbt/task/debug.py	82.40% <100.00%> (ø)
core/dbt/task/run.py	92.77% <100.00%> (ø)
core/dbt/tests/util.py	86.57% <100.00%> (ø)

... and 3 files with indirect coverage changes