Use staticmethod for PasswordStrength methods

[correctmost]

PR Description:

The methods don't need access to the class, so they don't need to be classmethods. This change reduces the number of 'bad return' warnings seen when running Pyright, Pyrefly, and ty.

Example warnings fixed:

# Pyright
archinstall/lib/models/users.py:61:13 - error: Type "Literal[PasswordStrength.STRONG]" is not assignable to return type "Self@PasswordStrength"
    Type "Literal[PasswordStrength.STRONG]" is not assignable to type "Self@PasswordStrength" (reportReturnType)

# Pyrefly
archinstall/lib/models/users.py:61:13-23: Returned type `Literal[PasswordStrength.STRONG]` is not assignable to declared return type `Self@PasswordStrength` [bad-return]

# ty
archinstall/lib/models/users.py:61:13: error[invalid-return-type] Return type does not match returned value: expected `Self@_check_password_strength`, found `Literal[PasswordStrength.STRONG]`

Tests and Checks

• I have tested the code!