`proxies` input argument is mutated

The input argument to proxies is mutated when environment proxy variables are present. See the reproduction steps.

This may be different than what users are expecting. It can lead to unexpected behavior when re-using the argument that was passed.

Expected Result

No mutation of the input argument:

>>> proxies = {'dummy': 'dummy'}
>>> os.environ['http_proxy'] = 'http://dummy'
>>> requests.get('https://python.org',proxies=proxies)
<Response [200]>
>>> proxies
{'dummy': 'dummy'}

Actual Result / Reproduction steps

>>> proxies = {'dummy': 'dummy'}
>>> os.environ['http_proxy'] = 'http://dummy'
>>> requests.get('https://python.org',proxies=proxies)
<Response [200]>
>>> proxies
{'dummy': 'dummy', 'http': 'http://dummy'}

System Information

$ python -m requests.help

{
  "chardet": {
    "version": null
  },
  "charset_normalizer": {
    "version": "2.0.12"
  },
  "cryptography": {
    "version": ""
  },
  "idna": {
    "version": "3.3"
  },
  "implementation": {
    "name": "CPython",
    "version": "3.10.4"
  },
  "platform": {
    "release": "5.10.102.1-microsoft-standard-WSL2",
    "system": "Linux"
  },
  "pyOpenSSL": {
    "openssl_version": "",
    "version": null
  },
  "requests": {
    "version": "2.27.1"
  },
  "system_ssl": {
    "version": "30000020"
  },
  "urllib3": {
    "version": "1.26.9"
  },
  "using_charset_normalizer": true,
  "using_pyopenssl": false
}

Start at the requests.get entry point and reproduce the issue with a proxies dictionary and an HTTP proxy environment variable. Verify that the caller's dictionary remains unchanged after the request, and add or update regression coverage for this behavior.