MacOS cannot use a proxy with HTTP basic authentication to access HTTPS

<!-- Summary. -->

## Expected Result
Displays the proxy IP address

<!-- What you expected. -->

## Actual Result

Traceback (most recent call last):
  File "/Users/cheney/code/python/tools/venv/lib/python3.8/site-packages/urllib3/connectionpool.py", line 696, in urlopen
    self._prepare_proxy(conn)
  File "/Users/cheney/code/python/tools/venv/lib/python3.8/site-packages/urllib3/connectionpool.py", line 964, in _prepare_proxy
    conn.connect()
  File "/Users/cheney/code/python/tools/venv/lib/python3.8/site-packages/urllib3/connection.py", line 371, in connect
    self._tunnel()
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.8/lib/python3.8/http/client.py", line 902, in _tunnel
    raise OSError("Tunnel connection failed: %d %s" % (code,
OSError: Tunnel connection failed: 407 Proxy Authentication Required

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/cheney/code/python/tools/venv/lib/python3.8/site-packages/requests/adapters.py", line 440, in send
    resp = conn.urlopen(
  File "/Users/cheney/code/python/tools/venv/lib/python3.8/site-packages/urllib3/connectionpool.py", line 755, in urlopen
    retries = retries.increment(
  File "/Users/cheney/code/python/tools/venv/lib/python3.8/site-packages/urllib3/util/retry.py", line 574, in increment
    raise MaxRetryError(_pool, url, error or ResponseError(cause))
urllib3.exceptions.MaxRetryError: HTTPSConnectionPool(host='ifconfig.me', port=443): Max retries exceeded with url: / (Caused by ProxyError('Cannot connect to proxy.', OSError('Tunnel connection failed: 407 Proxy Authentication Required')))

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/cheney/code/python/tools/test.py", line 17, in <module>
    res = requests.get("https://ifconfig.me", proxies=proxies)
  File "/Users/cheney/code/python/tools/venv/lib/python3.8/site-packages/requests/api.py", line 75, in get
    return request('get', url, params=params, **kwargs)
  File "/Users/cheney/code/python/tools/venv/lib/python3.8/site-packages/requests/api.py", line 61, in request
    return session.request(method=method, url=url, **kwargs)
  File "/Users/cheney/code/python/tools/venv/lib/python3.8/site-packages/requests/sessions.py", line 529, in request
    resp = self.send(prep, **send_kwargs)
  File "/Users/cheney/code/python/tools/venv/lib/python3.8/site-packages/requests/sessions.py", line 645, in send
    r = adapter.send(request, **kwargs)
  File "/Users/cheney/code/python/tools/venv/lib/python3.8/site-packages/requests/adapters.py", line 513, in send
    raise ProxyError(e, request=request)
requests.exceptions.ProxyError: HTTPSConnectionPool(host='ifconfig.me', port=443): Max retries exceeded with url: / (Caused by ProxyError('Cannot connect to proxy.', OSError('Tunnel connection failed: 407 Proxy Authentication Required')))

<!-- What happened instead. -->

## Reproduction Steps

When using a proxy that requires basic authentication to access an HTTPS (not HTTP) address, the system gives an error. The code is as follows:
```python
import requests

proxy_string = 'http://{}:{}@{}:{}'.format(username, password, proxyHost, proxyPort)
proxies = {"http": proxy_string, "https": proxy_string}
res = requests.get("https://ifconfig.me", proxies=proxies)
print(res.text)

```
It should be noted that the same code can run normally in Windows system, but there are problems with MacOS
## System Information

    $ python -m requests.help

```json
{
  "chardet": {
    "version": null
  },
  "charset_normalizer": {
    "version": "2.0.10"
  },
  "cryptography": {
    "version": ""
  },
  "idna": {
    "version": "3.3"
  },
  "implementation": {
    "name": "CPython",
    "version": "3.8.2"
  },
  "platform": {
    "release": "21.1.0",
    "system": "Darwin"
  },
  "pyOpenSSL": {
    "openssl_version": "",
    "version": null
  },
  "requests": {
    "version": "2.27.0"
  },
  "system_ssl": {
    "version": "20000000"
  },
  "urllib3": {
    "version": "1.26.7"
  },
  "using_charset_normalizer": true,
  "using_pyopenssl": false
}
```

<!-- This command is only available on Requests v2.16.4 and greater. Otherwise,
please provide some basic information about your system (Python version,
operating system, &c). -->


Please try 2.27.1, this should have been fixed in that release

Hi @ETOCheney, this is almost certainly a duplicate of #6027. This should be fixed in the latest release (2.27.1). I'm going to resolve this ticket for now, but please feel free to reopen if you find your issue persists after upgrading. Thanks!




> Hi @ETOCheney, this is almost certainly a duplicate of #6027. This should be fixed in the latest release (2.27.1). I'm going to resolve this ticket for now, but please feel free to reopen if you find your issue persists after upgrading. Thanks!

I tried to upgrade to latest release(2.27.1) and found that the problem has been fixed. Thanks!