Add `Levenshtein` and `nltk` to `requirements.txt`

[TheFloatingString]

Describe the bug

Using a Pipenv environment, I get a ModuleNotFoundError for Levenshtein and nltk when trying to run python -m jiant.scripts.benchmarks.benchmark_submission_formatter.

To Reproduce

1. Jiant v2.2.0
2. Windows 11, Pipenv environment
3. Config artifact: N/A

Expected behavior

python -m jiant.scripts.benchmarks.benchmark_submission_formatter should run without any ModuleNotFoundError errors.

pipenv install -r requirements.txt should install all the dependencies required for this project. However, in a Pipenv environment, the Levenshtein and nltk libraries are not included, as the requirements.txt file does not include Levenshtein and nltk.

As a result, running python -m jiant.scripts.benchmarks.benchmark_submission_formatter returns a ModuleNotFoundError for both dependencies.

While separately installing both Levenshtein and nltk is a quick fix, including both dependencies in the requirements.txt file in the root directory would solve these import errors when a user runs jiant in a new virtual environment where Levenshtein and nltk are not yet installed.

Additional context

Proposed additions to requirements.txt:

nltk==3.8.1
Levenshtein==0.23.0

Please not that I have not yet run tests using these versions, and that further review may be needed to ensure that all unit tests pass using these specific versions.

I'm happy to create a new branch and to add these two dependencies to the requirements* files, though please let me know which next steps would be the most ideal.