raise_for_status leaks HTTP basic auth

Calling `raise_for_status()` output (leaks) the HTTP basic auth password when raising an exception.

## Expected Result

```
>>> import requests
>>> r = requests.get("http://user:somepassw@httpbin.org/status/401")
>>> r.raise_for_status()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.7/site-packages/requests/models.py", line 940, in raise_for_status
    raise HTTPError(http_error_msg, response=self)
requests.exceptions.HTTPError: 401 Client Error: UNAUTHORIZED for url: http://user:<redacted>@httpbin.org/status/401
```
## Actual Result

Here we can see the HTTP password in the resulting exception (`somepassw` in our case)

```
>>> import requests
>>> r = requests.get("http://user:somepassw@httpbin.org/status/401")
>>> r.raise_for_status()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.7/site-packages/requests/models.py", line 940, in raise_for_status
    raise HTTPError(http_error_msg, response=self)
requests.exceptions.HTTPError: 401 Client Error: UNAUTHORIZED for url: http://user:somepassw@httpbin.org/status/401
```
## System Information

    $ python -m requests.help

```
{
  "chardet": {
    "version": "3.0.4"
  },
  "cryptography": {
    "version": ""
  },
  "idna": {
    "version": "2.8"
  },
  "implementation": {
    "name": "CPython",
    "version": "3.7.2"
  },
  "platform": {
    "release": "4.20.8-arch1-1-ARCH",
    "system": "Linux"
  },
  "pyOpenSSL": {
    "openssl_version": "",
    "version": null
  },
  "requests": {
    "version": "2.21.0"
  },
  "system_ssl": {
    "version": "1010101f"
  },
  "urllib3": {
    "version": "1.24.1"
  },
  "using_pyopenssl": false
}
```


For future reader, one way to fix this behaviour is to pass the basic auth by kwargs;

```
>>> import requests
>>> r = requests.get("http://httpbin.org/status/401", auth=("user", "passwd"))
>>> r.raise_for_status()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.7/site-packages/requests/models.py", line 940, in raise_for_status
    raise HTTPError(http_error_msg, response=self)
requests.exceptions.HTTPError: 401 Client Error: UNAUTHORIZED for url: http://httpbin.org/status/401
```

----

Still, seeing that python-requests send a `Authorization` header, it looks like we could do something for the original leak, as requests knows which parts of the URL is the authentification

```
>>> r = requests.get("http://user:password@httpbin.org/status/401")
>>> r.raise_for_status()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.7/site-packages/requests/models.py", line 940, in raise_for_status
    raise HTTPError(http_error_msg, response=self)
requests.exceptions.HTTPError: 401 Client Error: UNAUTHORIZED for url: http://user:password@httpbin.org/status/401
>>> r.request.headers
{'User-Agent': 'python-requests/2.21.0', 'Accept-Encoding': 'gzip, deflate', 'Accept': '*/*', 'Connection': 'keep-alive', 'Authorization': 'Basic dXNlcjpwYXNzd29yZA=='}
```