[pgAdmin] Fix incorrect import in setup.py

Hi,

First time posting here. I've attached a fix for an incorrect import
in `setup.py`. Trying to import from `pgadmin` before adding the root
directory to the system path results in import error, when the script
is invoked in any other way than running it directly. For example

$ /path/to/venv/bin/pip install pgadmin4
$ /path/to/venv/bin/python -m pgadmin4.setup
Traceback (most recent call last):
...
File "/path/to/venv/lib/python3.6/site-packages/pgadmin4/setup.py",
line 18, in <module>
from pgadmin.model import db, User, Version, ServerGroup, Server, \
ModuleNotFoundError: No module named 'pgadmin'

Please let me know what you think. Happy to address any feedback.

Regards,
Azad Salahlı