Add lower bound for aiohttp-cors + fix primer (#2231)

It appears sqlalchemy has recently reformatted their project with
Black 21.5b1.

Most of our dependencies have a lower bound and creating a test
environment with the oldest acceptable dependencies runs the full
Black test suite just fine. The only exception to this is aiohttp-cors.
It's unbounded and the oldest version 0.1.0 until 0.4.0 breaks the
test suite in such an old environment.

Failure with 0.1.0:

```
tests/test_blackd.py:10: in <module>
    import blackd
testenv/lib/python3.8/site-packages/blackd/__init__.py:12: in <module>
    import aiohttp_cors
testenv/lib/python3.8/site-packages/aiohttp_cors/__init__.py:29: in <module>
    from .urldispatcher_router_adapter import UrlDistatcherRouterAdapter
testenv/lib/python3.8/site-packages/aiohttp_cors/urldispatcher_router_adapter.py:27: in <module>
    class UrlDistatcherRouterAdapter(RouterAdapter):
testenv/lib/python3.8/site-packages/aiohttp_cors/urldispatcher_router_adapter.py:32: in UrlDistatcherRouterAdapter
    def route_methods(self, route: web.Route):
E   AttributeError: module 'aiohttp.web' has no attribute 'Route'
```

For 0.2.0:

```
tests/test_blackd.py:10: in <module>
    import blackd
testenv/lib/python3.8/site-packages/blackd/__init__.py:12: in <module>
    import aiohttp_cors
testenv/lib/python3.8/site-packages/aiohttp_cors/__init__.py:27: in <module>
    from .cors_config import CorsConfig
testenv/lib/python3.8/site-packages/aiohttp_cors/cors_config.py:24: in <module>
    from .urldispatcher_router_adapter import UrlDistatcherRouterAdapter
testenv/lib/python3.8/site-packages/aiohttp_cors/urldispatcher_router_adapter.py:27: in <module>
    class UrlDistatcherRouterAdapter(AbstractRouterAdapter):
testenv/lib/python3.8/site-packages/aiohttp_cors/urldispatcher_router_adapter.py:32: in UrlDistatcherRouterAdapter
    def route_methods(self, route: web.Route):
E   AttributeError: module 'aiohttp.web' has no attribute 'Route'
```

For 0.3.0:

```
ERROR: Cannot install aiohttp-cors==0.3.0 and aiohttp==3.6.0 because these package versions have conflicting dependencies.

The conflict is caused by:
    The user requested aiohttp==3.6.0
    aiohttp-cors 0.3.0 depends on aiohttp<=0.20.2 and >=0.18.0

To fix this you could try to:
1. loosen the range of package versions you've specified
2. remove package versions to allow pip attempt to solve the dependency conflict

ERROR: ResolutionImpossible: for help visit https://pip.pypa.io/en/latest/user_guide/#fixing-conflicting-dependencies
```