Add type hints to helpers and update DBF extraction tests

[zaneselvans]

PR Overview

This is just me being a little bit OCD.

• Added type hints to all remaining functions in pudl.helpers that didn't have them.
• Added schema compatability checks for FERC 6 and 60 DBF derived SQLite DBs.
• Use a context manager when dropping tables in pudl.helpers.drop_tables()

PR Checklist

• Merge the most recent version of the branch you are merging into (probably dev).
• All CI checks are passing. Run tests locally to debug failures
• Make sure you've included good docstrings.
• For major data coverage & analysis changes, run data validation tests
• Include unit tests for new functions and classes.
• Defensive data quality/sanity checks in analyses & data processing functions.
• Update the release notes and reference reference the PR and related issues.
• Do your own explanatory review of the PR to help the reviewer understand what's going on and identify issues preemptively.

[e-belfer]

Some other changes suggested while you're at it, and one non-blocking question. Otherwise looks good, presuming the code runs as expected.

[codecov]

Codecov Report

Patch coverage: 84.8% and project coverage change: -0.1% ⚠️

Comparison is base (f19f85a) 88.5% compared to head (73a0c79) 88.5%.
Report is 4 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##             dev   #2841     +/-   ##
=======================================
- Coverage   88.5%   88.5%   -0.1%     
=======================================
  Files         90      90             
  Lines      10139   10139             
=======================================
- Hits        8983    8982      -1     
- Misses      1156    1157      +1     

Files Changed	Coverage Δ
src/pudl/helpers.py	86.3% <84.8%> (-0.3%)	⬇️

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