Use difflib instead of python-Levenshtein for computing similarity ratio

The python-Levenshtein library has a GPLv2 license, meaning that the derived works must be available under the same license. Due to this, I, and presumably others, cannot use this library if we want a different license for works that use the word_forms library as a dependancy. (Thinking about this, it may mean that this particular library should also be GPLv2).

While looking for alternatives to this, I chanced upon Python’s own difflib library, and it’s SequenceMatcher.ratio() function. The output of this ratio, is exactly the same as the python-Levenshtein ratios. In fact, there is some overlap between the actual implementations of these libraries, as mentioned in the python-Levenshtein docs.

Code block to demonstrate this:

from difflib import SequenceMatcher
from Levenshtein import ratio

def sequence_matcher_ratio(a, b):
    return SequenceMatcher(None, a, b).ratio()

def compare_equality(a, b):
    print(sequence_matcher_ratio(a, b) == ratio(a, b))

def compare_print(a, b):
    print("Sequence Matcher Ratio: ", sequence_matcher_ratio(a, b))
    print("Levenshtein Ratio: ", ratio(a, b))

>>> compare_equality('continent', 'continence') 
True

>>> compare_print('continent', 'continence')
Sequence Matcher Ratio:  0.8421052631578947
Levenshtein Ratio:  0.8421052631578947

I propose we move to using SequenceMatcher, or some other library instead of the python-Levenshtein library.

I’m already doing this in my own fork, so I can raise a PR for this if needed.

[gutfeeling]

@sajal2692 Feel free to send the PR.

[gutfeeling]

Closing this as the PR is now merged.