[mypy] Fix type annotations for trie.py

[archaengel]

❯ mypy --ignore-missing-imports data_structures/trie
Success: no issues found in 2 source files

Related issue: #4052

Describe your change:

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms have a URL in its comments that points to Wikipedia or other similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[poyea]

Thank you for your pull request!🤩Let's make it --strict?

> mypy . --ignore-missing-imports --strict
trie.py:10: error: Function is missing a return type annotation
trie.py:10: note: Use "-> None" if function does not return a value
trie.py:14: error: Function is missing a return type annotation
trie.py:23: error: Function is missing a return type annotation
trie.py:32: error: Call to untyped function "TrieNode" in typed context
trie.py:49: error: Function is missing a return type annotation
trie.py:56: error: Function is missing a return type annotation
trie.py:78: error: Function is missing a return type annotation
trie.py:92: error: Function is missing a return type annotation
trie.py:94: error: Call to untyped function "TrieNode" in typed context
trie.py:115: error: Function is missing a return type annotation
trie.py:115: note: Use "-> None" if function does not return a value
trie.py:116: error: Call to untyped function "test_trie" in typed context
trie.py:119: error: Function is missing a return type annotation
trie.py:119: note: Use "-> None" if function does not return a value
trie.py:123: error: Call to untyped function "test_trie" in typed context
trie.py:127: error: Call to untyped function "main" in typed context
Found 14 errors in 1 file (checked 3 source files)

[archaengel]

Thanks for the review! 🙏🏽 That sgtm, done.