Django-postgrespool2

Fixed
- Disable psycopg2 parsing JSONField value in Django 3.1+ ([29](https://github.com/lcd1232/django-postgrespool2/pull/29))

Fixed
- Fixed timezone aware ([27](https://github.com/lcd1232/django-postgrespool2/pull/27))

Added
- Support django 2.2, 3.0 and 3.1
- Support python 3.6, 3.7, 3.8 and 3.9
Removed
- Remove support django 1.8, 1.9, 2.0, 2.1
- Remove support python 3.4 and 3.5

Fixed
- Fixed installation on python 3.7, thanks [chickahoona](https://github.com/lcd1232/django-postgrespool2/pull/16)

Added
- New setting `DATABASE_POOL_CLASS`, thanks [mozartilize](https://github.com/mozartilize)
Changed
- Rewrite internal logic of library, thanks [mozartilize](https://github.com/mozartilize)
Removed
- Remove support python 3.3
- Remove support django 1.7
Fixed
- Add missed backend `django_postgrespool2.postgis`

Added
- Now you can specify backend for engine. Example:
python
DATABASES = {
'default': {
'ENGINE': 'django_postgrespool2.psycopg2',
'HOST': 'localhost',
'PORT': '5432',
'USER': 'test',
'PASSWORD': 'test',
}
}

Available backends: **django_postgrespool2.psycopg2**, **django_postgrespool2.postgis**. By default it using psycopg2 as backend engine.
Changed
- Update to Django 2.0
- Fix error with pre_ping option
- Fix error with __version__