Only prefix a slash on the api_gateway_base_path if needed

[dspatoulas]

Only appends a prefix to the api_gw_base_path if it's not there already to avoid duplicate prefix slashes.

Issue: #137

Intended Design Change

api_gw_base_path = "prod"
api_gw_base_path = "/prod"

def lambda_handler(event, context):  # pragma: no cover
    handler = mangum.Mangum(api, api_gateway_base_path=settings.api_gw_base_path.lstrip("/"))  #137 Avoids the need for all consumers to call .lstrip("/") on their base path str
    response = handler(event, context)
    return response

[jordaneremieff]

@dspatoulas Looks like a good improvement, thanks!

Could you code format the adapter file with black and push again? Looks like the line-length of the change is just sightly too long, so this will cause the build to fail. Once this is resolved I'll merge it in.