chore: add optional asyncpg dependency to enable asyncio support (#30)

Author: hadrien

poetry.lock

@@ -56,6 +56,7 @@ pytest-asyncio = { version = "^0.14.0", optional = true}
 pytest-cov = { version = "^2.10.1", optional = true}
 pytest-watch = { version = "^4.2.0", optional = true}
 tox = { version = "^3.23.0", optional = true}
+asyncpg = {version = "^0.22.0", optional = true}
 
 [tool.poetry.extras]
 # Test dependencies as extras so they can be set as extras in tox config
@@ -76,6 +77,7 @@ tests = [
     "pytest-watch",
     "tox",
 ]
+asyncio = ["asyncpg"]
 
 
 [build-system]