#896 Restrict including routers with strings

[JrooTJunior]

Description

Fixes #896

Type of change

Please delete options that are not relevant.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[github-actions]

✔️ Changelog found.

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[codecov]

Codecov Report

Merging #897 (d8de1cc) into dev-3.x (4fb77a3) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           dev-3.x      #897   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          301       300    -1     
  Lines         8192      8172   -20     
=========================================
- Hits          8192      8172   -20     

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

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Impacted Files	Coverage Δ
aiogram/dispatcher/router.py	100.00% <ø> (ø)