`cert` parameter does not accept a single `pathlib.Path` object as argument

requests seems to have special handling for when the cert parameter is a single str, but does not extend the same special handling to pathlib.Path, meaning currently you either have to do cert=str(path) or cert=(path, path).

Expected Result

Path objects should be handled just like str.

Actual Result

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/nyuszika7h/test/.venv/lib/python3.9/site-packages/requests/api.py", line 75, in get
    return request('get', url, params=params, **kwargs)
  File "/home/nyuszika7h/test/.venv/lib/python3.9/site-packages/requests/api.py", line 61, in request
    return session.request(method=method, url=url, **kwargs)
  File "/home/nyuszika7h/test/.venv/lib/python3.9/site-packages/requests/sessions.py", line 542, in request
    resp = self.send(prep, **send_kwargs)
  File "/home/nyuszika7h/test/.venv/lib/python3.9/site-packages/requests/sessions.py", line 655, in send
    r = adapter.send(request, **kwargs)
  File "/home/nyuszika7h/test/.venv/lib/python3.9/site-packages/requests/adapters.py", line 416, in send
    self.cert_verify(conn, request.url, verify, cert)
  File "/home/nyuszika7h/test/.venv/lib/python3.9/site-packages/requests/adapters.py", line 243, in cert_verify
    conn.cert_file = cert[0]
TypeError: 'PosixPath' object is not subscriptable

Reproduction Steps

from pathlib import Path

import requests

requests.get('https://example.com', cert=Path('cert.pem'))

System Information

$ python -m requests.help

{
  "chardet": {
    "version": null
  },
  "charset_normalizer": {
    "version": "2.0.5"
  },
  "cryptography": {
    "version": ""
  },
  "idna": {
    "version": "3.2"
  },
  "implementation": {
    "name": "CPython",
    "version": "3.9.2"
  },
  "platform": {
    "release": "5.10.0-8-amd64",
    "system": "Linux"
  },
  "pyOpenSSL": {
    "openssl_version": "",
    "version": null
  },
  "requests": {
    "version": "2.26.0"
  },
  "system_ssl": {
    "version": "101010bf"
  },
  "urllib3": {
    "version": "1.26.6"
  },
  "using_charset_normalizer": true,
  "using_pyopenssl": false
}

Start at requests/adapters.py, specifically the cert_verify entry point shown in the traceback, and compare its handling of a single cert value with the reported string behavior. Re-run the Path('cert.pem') reproduction and consider the issue done when a single pathlib.Path is accepted like a string without the reported TypeError.