How to write requirements.txt?

Question

Docker

There was an example in the docker documentation:
Django>=2.1,<3.0
psycopg2>=2.7,<3.0
Why are there two versions?

Answer 1

Django minimum version 2.1 and maximum version 2.9999999 but not 3.0
psycopg2 minimum 2.7 but maximum 2.99999 to 3.0 but not 3.0