Replaced including venv/ directory with requirements.txt

[ajstensland]

When cloning the ScorchingShade/Encryptor repository, 32 out of 38MB of disk space used are from the venv/ directory.

Generally, it is best practice to exclude virtual environment directories from repositories, as discussed here and here. Thus, I've replaced the venv/ directory with a requirements.txt file as described here.

I also noticed that the virtual environment does not include progress, which you import Bar from. This results in an ImportError due to progress not being installed. I found that progress wasn't mentioned in the README.md.

To fix these issues, I've done the following:

1. Included a requirements.txt that instructs pip to install pycryptodome as well as progress
2. Edited README.md to use the requirements.txt, and added progress as a declared requirement
3. Removed the venv/ directory

This makes the setup process more flexible, and reduces the repository size from 32MB to 5.8MB.

Let me know if anything needs changing!