The built-in SocketMode client fails to establish WS connections via proxy server

[Lordshinjo]

Reproducible in:

The Slack SDK version

slack-sdk==3.13.0

Python runtime version

3.6.13

OS info

ProductName:	macOS
ProductVersion:	11.6.3
BuildVersion:	20G415
Darwin Kernel Version 20.6.0: Wed Jan 12 22:22:45 PST 2022; root:xnu-7195.141.19~2/RELEASE_ARM64_T8101

Steps to reproduce:

1. Pass an authenticated proxy url containing url-encoded characters (e.g. http://username:pass%2Fword@example.com) to a synchronous slack_sdk.socket_mode.SocketModeClient, either via HTTPS_PROXY env variable or proxy argument.
2. Connect to the Slack API.

Expected result:

The connection through the proxy is successful.

Actual result:

The connection fails with the following error:

Traceback (most recent call last):
  File "[redacted]/lib/python3.6/site-packages/slack_sdk/socket_mode/builtin/connection.py", line 116, in connect
    trace_enabled=self.trace_enabled,
  File "[redacted]/lib/python3.6/site-packages/slack_sdk/socket_mode/builtin/internals.py", line 81, in _establish_new_socket_connection
    f"Failed to connect to the proxy (proxy: {proxy}, connect status code: {status})"
Exception: Failed to connect to the proxy (proxy: http://[redacted]:[redacted with url encoded chars]@[redacted url]:80, connect status code: 407)

The documentation for urllib.parse.urlparse mentions that % escapes are not expanded, and username and password should not be url-encoded when they are passed as part of a Basic authentication.

As far as I understand the issue is simply that the username and password should be url-decoded before being base64'd in the following code:

python-slack-sdk/slack_sdk/socket_mode/builtin/internals.py

Lines 60 to 64 in 77e906a

	if parsed_proxy.username is not None and parsed_proxy.password is not None:
	# In the case where the proxy is "http://{username}:{password}@{hostname}:{port}"
	raw_value = f"{parsed_proxy.username}:{parsed_proxy.password}"
	auth = b64encode(raw_value.encode("utf-8")).decode("ascii")
	message.append(f"Proxy-Authorization: Basic {auth}")

(As a side note, printing the full proxy info including the password in the exception is not so great, but that's another issue.)

[seratch]

HI @Lordshinjo, thanks a lot for taking the time to report this issue! We'll fix this issue in the next release. If you're interested in fixing this issue, your pull request contributions are always welcomed 👋