[llvm] [Workflow] make code-format-helper.py mypy-safe (NFC) (PR #69691)

[Ryan Prichard via llvm-commits]

rprichard wrote:

Previously mypy was complaining on this file:
```
$ mypy code-format-helper.py 
code-format-helper.py:29: error: Bracketed expression "[...]" is not valid as a type  [valid-type]
code-format-helper.py:29: note: Did you mean "List[...]"?
code-format-helper.py:36: error: "FormatHelper" has no attribute "friendly_name"  [attr-defined]
code-format-helper.py:44: error: "FormatHelper" has no attribute "instructions"  [attr-defined]
code-format-helper.py:90: error: "FormatHelper" has no attribute "friendly_name"  [attr-defined]
code-format-helper.py:94: error: Bracketed expression "[...]" is not valid as a type  [valid-type]
code-format-helper.py:94: note: Did you mean "List[...]"?
code-format-helper.py:123: error: Bracketed expression "[...]" is not valid as a type  [valid-type]
code-format-helper.py:123: note: Did you mean "List[...]"?
code-format-helper.py:132: error: Bracketed expression "[...]" is not valid as a type  [valid-type]
code-format-helper.py:132: note: Did you mean "List[...]"?
code-format-helper.py:135: error: Return value expected  [return-value]
code-format-helper.py:162: error: Bracketed expression "[...]" is not valid as a type  [valid-type]
code-format-helper.py:162: note: Did you mean "List[...]"?
code-format-helper.py:171: error: Bracketed expression "[...]" is not valid as a type  [valid-type]
code-format-helper.py:171: note: Did you mean "List[...]"?
code-format-helper.py:174: error: Return value expected  [return-value]
Found 11 errors in 1 file (checked 1 source file)
```


https://github.com/llvm/llvm-project/pull/69691