Unicode surrogates in POST data lead to exception

<!-- Summary. -->

In requests 2.26.0 with Python 3, when passing unicode strings with surrogate characters as POST data (either directly as a `str` or as part of a `dict`), sending the request throws an exception.

## Expected Result

The library should not throw an exception, even when fed with unvalidated user input.

<!-- What you expected. -->

## Actual Result / Reproduction Steps

```
>>> import requests
>>> requests.__version__
'2.26.0'

>>> requests.post('https://example.invalid', data={'name': 'test\udced\udcb3\udc83.pdf'})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/user/.local/lib/python3.7/site-packages/requests/api.py", line 117, in post
    return request('post', url, data=data, json=json, **kwargs)
  File "/home/user/.local/lib/python3.7/site-packages/requests/api.py", line 61, in request
    return session.request(method=method, url=url, **kwargs)
  File "/home/user/.local/lib/python3.7/site-packages/requests/sessions.py", line 528, in request
    prep = self.prepare_request(req)
  File "/home/user/.local/lib/python3.7/site-packages/requests/sessions.py", line 466, in prepare_request
    hooks=merge_hooks(request.hooks, self.hooks),
  File "/home/user/.local/lib/python3.7/site-packages/requests/models.py", line 319, in prepare
    self.prepare_body(data, files, json)
  File "/home/user/.local/lib/python3.7/site-packages/requests/models.py", line 515, in prepare_body
    body = self._encode_params(data)
  File "/home/user/.local/lib/python3.7/site-packages/requests/models.py", line 104, in _encode_params
    v.encode('utf-8') if isinstance(v, str) else v))
UnicodeEncodeError: 'utf-8' codec can't encode characters in position 4-6: surrogates not allowed

>>> requests.post('https://example.com/', 'test\udced\udcb3\udc83.pdf')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/user/.local/lib/python3.7/site-packages/requests/api.py", line 117, in post
    return request('post', url, data=data, json=json, **kwargs)
  File "/home/user/.local/lib/python3.7/site-packages/requests/api.py", line 61, in request
    return session.request(method=method, url=url, **kwargs)
  File "/home/user/.local/lib/python3.7/site-packages/requests/sessions.py", line 542, in request
    resp = self.send(prep, **send_kwargs)
  File "/home/user/.local/lib/python3.7/site-packages/requests/sessions.py", line 655, in send
    r = adapter.send(request, **kwargs)
  File "/home/user/.local/lib/python3.7/site-packages/requests/adapters.py", line 449, in send
    timeout=timeout
  File "/home/user/.local/lib/python3.7/site-packages/urllib3/connectionpool.py", line 706, in urlopen
    chunked=chunked,
  File "/home/user/.local/lib/python3.7/site-packages/urllib3/connectionpool.py", line 394, in _make_request
    conn.request(method, url, **httplib_request_kw)
  File "/home/user/.local/lib/python3.7/site-packages/urllib3/connection.py", line 239, in request
    super(HTTPConnection, self).request(method, url, body=body, headers=headers)
  File "/usr/lib/python3.7/http/client.py", line 1260, in request
    self._send_request(method, url, body, headers, encode_chunked)
  File "/usr/lib/python3.7/http/client.py", line 1305, in _send_request
    body = _encode(body, 'body')
  File "/usr/lib/python3.7/http/client.py", line 174, in _encode
    (name.title(), data[err.start:err.end], name)) from None
UnicodeEncodeError: 'latin-1' codec can't encode characters in position 4-6: Body ('\udced\udcb3\udc83') is not valid Latin-1. Use body.encode('utf-8') if you want to send it encoded in UTF-8.
```

## System Information

    $ python -m requests.help

```json
{
  "chardet": {
    "version": "3.0.4"
  },
  "charset_normalizer": {
    "version": "2.0.9"
  },
  "cryptography": {
    "version": ""
  },
  "idna": {
    "version": "3.3"
  },
  "implementation": {
    "name": "CPython",
    "version": "3.7.3"
  },
  "platform": {
    "release": "4.19.0-17-amd64",
    "system": "Linux"
  },
  "pyOpenSSL": {
    "openssl_version": "",
    "version": null
  },
  "requests": {
    "version": "2.26.0"
  },
  "system_ssl": {
    "version": "1010104f"
  },
  "urllib3": {
    "version": "1.26.7"
  },
  "using_charset_normalizer": false,
  "using_pyopenssl": false
}
```
-----
In [requests/models.py](https://github.com/psf/requests/blob/39d0fdd9096f7dceccbc8f82e1eda7dd64717a8e/requests/models.py), there are several calls like `some_string.encode('utf-8')`. I suppose changing them to `some_string.encode('utf-8', 'surrogatepass')` would at least fix the first reproduction case. Not sure about the second, since the exception occurs in http.client there.