[ENH] Updated label_encode to use pandas factorize

[nvamsikrishna05]

PR Description

Please describe the changes proposed in the pull request:

• Updated label_encode to use pandas factorize implementation internally replacing scikit-learn LabelEncoder
• Deprecation warning has been added to label_encode
• Removed scikit-learn as a dependency from environment-dev.yml and base.in files
• Regenerated base.txt using pip-compile from base.in after removing scikit-learn

This PR resolves #834

PR Checklist

Please ensure that you have done the following:

1. PR in from a fork off your branch. Do not PR from <your_username>:dev, but rather from <your_username>:<feature-branch_name>.

2. If you're not on the contributors list, add yourself to AUTHORS.rst.

3. Add a line to CHANGELOG.md under the latest version header (i.e. the one that is "on deck") describing the contribution.
   • Do use some discretion here; if there are multiple PRs that are related, keep them in a single line.

Automatic checks

There will be automatic checks run on the PR. These include:

• Building a preview of the docs on Netlify
• Automatically linting the code
• Making sure the code is documented
• Making sure that all tests are passed
• Making sure that code coverage doesn't go down.

Relevant Reviewers

Please tag maintainers to review.

• @ericmjl
• @samukweku
• @loganthomas

[nvamsikrishna05]

@ericmjl
Will raise another PR for the Following:

• Add new factorize_columns function and it's associated tests
• Update the warning message for label_encode to guide users to use factorize_columns.

[ericmjl]

Looking at the code changes, I have no comments to make. @nvamsikrishna05 please do the honours of merging!