upgrade.sh does not require Python 3.8

[markkuleinio]

NetBox version

v3.2.1

Python version

3.8

Steps to Reproduce

1. Run upgrade.sh on a system with Python 3.7 only

Expected Behavior

upgrade.sh should complain about the minimum required Python version (3.8 for NetBox 3.2)

Observed Behavior

# ./upgrade.sh
Using Python 3.7.3
...
ERROR: Could not find a version that satisfies the requirement Django==4.0.4

so there is no direct error message about the Python version.

In the start of upgrade.sh there is a Python version check but it only checks for 3.7+ while Python 3.8+ is required for NetBox 3.2.

[markkuleinio]

PR ready to be submitted if issue is accepted.

[jeremystretch]

We do have the logic in there but it currently only checks for v3.7+, so it should be an easy tweak. Thanks!