You can use pipreqs to automatically generate a requirements.txt file based on the import statements that the Python script(s) contain. To use pipreqs, assuming that you are in the directory where example.py is located:
pip install pipreqs
pipreqs .
It will generate the following requirements.txt file:
requests==2.23.0
beautifulsoup4==4.9.1
which you can install with:
pip install -r requirements.txt