proxy argument is unsupported in sync client since v2.6

[seratch]

Description

Since v2.6.0, run_async=False mode WebClient hasn't been working with proxy option.

What type of issue is this? (place an x in one of the [ ])

• bug
• enhancement (feature request)
• question
• documentation related
• testing related
• discussion

Requirements (place an x in each of the [ ])

• I've read and understood the Contributing guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've searched for any related issues and avoided creating a duplicate issue.

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Bug Report

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Reproducible in:

• slackclient version: v2.6.0+
• python version: any
• OS version(s): any

Steps to reproduce:

import os
from slack import WebClient

client = WebClient(
    token=os.environ["SLACK_API_TOKEN"],
    proxy="http://localhost:9000"
)
response = client.auth_test()
logger.info(f"sync response: {response}")

Expected result:

urllib.error.URLError: <urlopen error [Errno 61] Connection refused>

Actual result:

WebClient ignores the proxy setting. It successfully sends a request to Slack.