[-] Be able to run tox locally

[timjklein36]

What

This PR makes it possible to run the full tox suite on a developer's local machine.

Why

This is really useful for making sure that developing changes do not break tests when run against older versions of Django or Python. This is also a good sanity check before submitting PRs, since it is exactly the same suite of tests that will run via CI.

How

To facilitate running tox locally, the only needed change was to add PGUSER=postgres to the setenv section of the tox.ini. This was defaulting to the developer's username so the tests could not connect to the DB that was running (since postgres is the usual default for DB user name).

Along with this, a new script was added (./postgres-docker) that makes it easy for a developer to spin up a PostgreSQL instance with all the proper extensions installed in order to run the full postgis test suite that some of the tox environment targets use.

Therefore, running tox locally can look something like this:

$ ./postgres-docker

Checking dependencies [docker]...
Starting 'model-bakery-postgres'...
4d2aa3b2c1d26cc5fbbf093c7548e804f4229c1a4955a118f64616ff4cd37e76
Waiting for DB container...
CREATE EXTENSION
CREATE EXTENSION
CREATE EXTENSION

$ tox

GLOB sdist-make: /home/tim/projects/model_bakery/setup.py
py36-django22-postgresql inst-nodeps: /home/tim/projects/model_bakery/.tox/.tmp/package/1/model_bakery-1.3.2.zip
py36-django22-postgresql installed: attrs==21.2.0,Django==2.2,importlib-metadata==4.8.1,iniconfig==1.1.1,model-bakery==1.3.2,packaging==21.0,Pillow==8.3.2,pluggy==1.0.0,psycopg2-binary==2.9.1,py==1.10.0,pyparsing==2.4.7,pytest==6.2.5,pytest-django==4.4.0,pytz==2021.3,sqlparse==0.4.2,toml==0.10.2,typing-extensions==3.10.0.2,zipp==3.6.0
py36-django22-postgresql run-test-pre: PYTHONHASHSEED='4241793774'
py36-django22-postgresql run-test: commands[0] | pytest
=============================================================================================== test session starts ===============================================================================================
platform linux -- Python 3.6.9, pytest-6.2.5, py-1.10.0, pluggy-1.0.0
cachedir: .tox/py36-django22-postgresql/.pytest_cache
rootdir: /home/tim/projects/model_bakery, configfile: setup.cfg
plugins: django-4.4.0

... (REST OF OUTPUT TRUNCATED)

Note: there already existed a postgis-tests.sh script which was refactored a bit to utilize the new postgres-docker script for the same purpose that it was handling itself before.

[github-actions]

@timjklein36 your pull request is missing a changelog!

[timjklein36]

@berinhard I don't think this PR needs a CHANGELOG update, since it is just devops stuff.