Rename default branch to `main`

[jakob-keller]

Describe the bug
Currently this repo's default branch is master. GitHub now defaults to main as the default branch for new repos. We should follow that convention.

Checklist

• I have reproduced in environment where pip check passes without errors
• I have provided pip freeze results
• I have provided sample code or detailed way to reproduce
• I have tried the same code in botocore to ensure this is an aiobotocore specific issue
• I have tried similar code in aiohttp to ensure this is is an aiobotocore specific issue
• I have checked the latest and older versions of aiobotocore/aiohttp/python to see if this is a regression / injection

pip freeze results

Environment:

• Python Version: [e.g. 3.6]
• OS name and version: [e.g. darwin stretch]

Additional context
Add any other context about the problem here.