Bump up versions

• Update websockets version to >=9.1,<11.0

Bump up versions

• Update minimum Python Requirement to 3.7, since Sanic dropped Python 3.6 support
• Update httpx version to >=0.18.1
• Update websockets version to >=9.1,<10.0

Fix Incompatible Issue with Sanic 21.3.0

Fix incompatible issue with Sanic 21.3.0, #50

Replace aiohttp with httpx.

Replace heavy aiohttp package by lightweight async client, httpx. aiohttp includes implementation for client/server while pytest-sanic uses only ClientSession functionality. As a result aiohttp takes more disk space ~4.0M comparing to ~450kB (httpx). Also, aiohttp has more auxiliary packages.

Incompatible changes

• test_client.get/post/head/delete now returns httpx.Response, instead of aiohttp.web.Response.