Add requirements.txt for Python dependencies

[charishma1407]

Purpose:
This PR adds requirements.txt file for installing python dependencies.
Updated README.md as per the requirements in Ubuntu and MacOs.

Checklist

1. Does this PR change the input/output behaviour of a cryptographic algorithm (i.e., does it change known answer test values)? (If so, a version bump will be required from x.y.z to x.(y+1).0.)
   No This PR is a document and dependency change only
2. Does this PR change the list of algorithms available -- either adding, removing, or renaming? Does this PR otherwise change an API? (If so, PRs in fully supported downstream projects dependent on these, i.e., [oqs-provider]
   No

Generative AI
This contribution is not made with the help of a generative AI.

[dstebila]

Hi @charishma1407, thanks for looking at this. As far as I can tell, on Ubuntu the preferred way to install packages is via apt, rather than pip3. Is there a reason to do it the way you've proposed?

[charishma1407]

You're right — apt is the preferred and safer way to install packages on Ubuntu as it avoids conflicts with system-managed Python. I have reverted the Ubuntu install step back to use apt and keep requirements.txt only for macOS and other platforms where pip3 is the standard approach.
I thought of making it to generic so I made pip3 for both macOS and ubuntu.

[xuganyu96]

Hi @charishma1407, thanks for looking at this. As far as I can tell, on Ubuntu the preferred way to install packages is via apt, rather than pip3. Is there a reason to do it the way you've proposed?

With pip we can pin exact version and verify sha-256 checksum. I would actually argue against installing pytest (and its plugins) and other script dependencies via system package manager precisely because apt versions might drift way from pip versions, which then makes CI image dependencies drift from local development setup. Especially for local setup, a virtual environment (i.e. venv, virtualenv, conda) should be the strongly recommended default.

[coveralls]

coverage: 81.416% (-3.5%) from 84.927% — charishma1407:charishma_requirements_txt into open-quantum-safe:main