[Mypy] fix other/least_recently_used

[spazm]

Describe your change:

• fix bug in dq_removal code. Needed to pass key to .remove() rather than index.
• remove incorrect annotation to make the __init__ an abstract function.
• Annotate all functions and class Generic[T]
• Expand example to show creating a LRUCache over concrete and mixed types.
• Adds doctest to LRUCache class
• Adds assert test in main.

Previously opened as #5656

Closes: #4052

Before

% git checkout master
Your branch is up to date with 'origin/master'.

% mypy --ignore-missing-imports --install-types least_recently_used.py
least_recently_used.py:57: error: Cannot instantiate abstract class "LRUCache" with abstract attribute "__init__"
Found 1 error in 1 file (checked 1 source file)

% mypy --strict least_recently_used.py
least_recently_used.py:27: error: Function is missing a type annotation
least_recently_used.py:33: error: Unsupported right operand type for in ("object")
least_recently_used.py:34: error: Argument 1 to "len" has incompatible type "object"; expected "Sized"
least_recently_used.py:35: error: "object" has no attribute "pop"
least_recently_used.py:36: error: "object" has no attribute "remove"
least_recently_used.py:39: error: Need type annotation for "key"
least_recently_used.py:39: error: Argument 1 to "enumerate" has incompatible type "object"; expected "Iterable[<nothing>]"
least_recently_used.py:43: error: "object" has no attribute "remove"
least_recently_used.py:45: error: "object" has no attribute "appendleft"
least_recently_used.py:46: error: "object" has no attribute "add"
least_recently_used.py:48: error: Function is missing a return type annotation
least_recently_used.py:48: note: Use "-> None" if function does not return a value
least_recently_used.py:52: error: "object" has no attribute "__iter__"; maybe "__str__" or "__dir__"? (not iterable)
least_recently_used.py:57: error: Cannot instantiate abstract class "LRUCache" with abstract attribute "__init__"
least_recently_used.py:58: error: Call to untyped function "refer" in typed context
least_recently_used.py:59: error: Call to untyped function "refer" in typed context
least_recently_used.py:60: error: Call to untyped function "refer" in typed context
least_recently_used.py:61: error: Call to untyped function "refer" in typed context
least_recently_used.py:62: error: Call to untyped function "refer" in typed context
least_recently_used.py:63: error: Call to untyped function "refer" in typed context
least_recently_used.py:64: error: Call to untyped function "display" in typed context
Found 20 errors in 1 file (checked 1 source file)

After

% git checkout mypy-fix-other-least_recently_used
Switched to branch 'mypy-fix-other-least_recently_used'
Your branch is up to date with 'origin/mypy-fix-other-least_recently_used'.

% mypy --strict least_recently_used.py
Success: no issues found in 1 source file

• 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}.

[cclauss]

LGTM. Thanks for doing this and thanks for making us mypy compliant.