Fix URL Formatting to Prevent Double Slashes in Base URL and API Method Combination

[HTSagara]

(Filling out the following details about bugs will help us solve your issue sooner.)
I just notice that my last PR #1594 cause an error after I added the logic in the legacy_base_client.py due to a double slash in the api_url when concatenating the base_url with the api_method.py.

Reproducible in:

>pip freeze | grep slack
python --version
sw_vers && uname -v # or `ver`

Python 3.12.7
ProductName:            macOS
ProductVersion:         14.6.1
BuildVersion:           23G93
Darwin Kernel Version 23.6.0: Mon Jul 29 21:13:04 PDT 2024; root:xnu-10063.141.2~1/RELEASE_ARM64_T6020

>./scripts/run_validation.sh

The Slack SDK version

(Paste the output of pip freeze | grep slack)

Python runtime version

(Paste the output of python --version)
Python 3.12.7

OS info

(Paste the output of sw_vers && uname -v on macOS/Linux or ver on Windows OS)

Steps to reproduce:

(Share the commands to run, source code, and project settings (e.g., setup.py))

1. source env_3.9.6/bin/activate
2. ./scripts/run_validation.sh

Expected result:

Expected to pass all the tests

Actual result:

Test test_issue_690_oauth_v2_access from tests/web/test_web_client.py fails

Proposed solution:

Update the _get_url(base_url: str, api_method: str) -> str ensures that a properly formatted absolute URL is constructed from a base_url and an api_method, handling edge cases where redundant slashes might cause incorrect URLs.

def _get_url(base_url: str, api_method: str) -> str:
    """Joins the base Slack URL and an API method to form an absolute URL.

    Args:
        base_url (str): The base URL
        api_method (str): The Slack Web API method. e.g. 'chat.postMessage'

    Returns:
        The absolute API URL.
            e.g. 'https://slack.com/api/chat.postMessage'
    """
    if base_url.endswith("/") and api_method.startswith("/"):
        base_url = base_url.rstrip("/")
        
    return urljoin(base_url, api_method)

Apologies for the inconvenience and my lack of attention, I'm sending a PR to fix this issue ASAP.

Requirements

For general questions/issues about Slack API platform or its server-side, could you submit questions at https://my.slack.com/help/requests/new instead. 🙇

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[seratch]

The changes were reverted by #1597