Improve typing in import_functions, export_functions

[soininen]

Improve typing. No functional changes.

No associated issue.

Checklist before merging

• Documentation (also in Toolbox repo) is up-to-date
• Release notes have been updated
• Unit tests have been added/updated accordingly
• Code has been formatted by black & isort
• Unit tests pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.71%. Comparing base (1d8999c) to head (b8dab5d).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #609      +/-   ##
==========================================
+ Coverage   85.68%   85.71%   +0.02%     
==========================================
  Files          82       82              
  Lines       11076    11094      +18     
  Branches     1626     1626              
==========================================
+ Hits         9491     9509      +18     
  Misses       1228     1228              
  Partials      357      357              

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