Custom SSL context is ignored when using proxy

## Summary

I'm using a custom HTTPAdapter to bypass TLS fingerprinting. Without a proxy, everything works fine. However, when using a proxy, the custom cipher settings appear to be ignored and it falls back to the default `requests` SSL context.

I have confirmed this with multiple non-intercepting HTTP and HTTPS proxies, and fact that the fingerprint is identical to the default `requests` one every time (despite Python or `requests` not being involved in the proxy software at all) confirms that this is not an issue with my proxy.

## Expected Result

The custom SSL context settings should be respected.

## Actual Result

The custom SSL context settings are ignored and the defaults are used.

## Reproduction Steps

```python
#!/usr/bin/env python3

import ssl

import requests


PROXY = 'REDACTED'


class TLSFPBypassAdapter(requests.adapters.HTTPAdapter):
    def init_poolmanager(self, *args, **kwargs):
        print('TLSFPBypassAdapter.init_poolmanager called')
        ctx = ssl.create_default_context()
        ctx.set_ciphers('ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-SHA256:AES256-SHA')
        ctx.check_hostname = False  # For some reason, this is needed to avoid a verification error
        kwargs['ssl_context'] = ctx
        return super().init_poolmanager(*args, **kwargs)


def get_tls_fingerprint(session):
    return session.get('https://tls.browserleaks.com/json').json()['ja3_hash']


print('Default SSL context:')
session1 = requests.Session()
print(get_tls_fingerprint(session1))

print()

print('Default SSL context + proxy:')
session2 = requests.Session()
session2.proxies.update({'all': PROXY})
print(get_tls_fingerprint(session2))

print()

print('Custom SSL context:')
session3 = requests.Session()
session3.mount('http://', TLSFPBypassAdapter())
session3.mount('https://', TLSFPBypassAdapter())
print(get_tls_fingerprint(session3))

print()

print('Custom SSL context + proxy:')
session4 = requests.Session()
session4.mount('http://', TLSFPBypassAdapter())
session4.mount('https://', TLSFPBypassAdapter())
session4.proxies.update({'all': PROXY})
print(get_tls_fingerprint(session4))
```

Output:
```
Default SSL context:
e85f4ba10180c530d8c0dc5577f88413

Default SSL context + proxy:
e85f4ba10180c530d8c0dc5577f88413

Custom SSL context:
TLSFPBypassAdapter.init_poolmanager called
TLSFPBypassAdapter.init_poolmanager called
5a923e30925d6e3fe0dd6ff098905a01

Custom SSL context + proxy:
TLSFPBypassAdapter.init_poolmanager called
TLSFPBypassAdapter.init_poolmanager called
e85f4ba10180c530d8c0dc5577f88413
```

## System Information

    $ python -m requests.help

```json
{
  "chardet": {
    "version": null
  },
  "charset_normalizer": {
    "version": "2.0.12"
  },
  "cryptography": {
    "version": ""
  },
  "idna": {
    "version": "3.3"
  },
  "implementation": {
    "name": "CPython",
    "version": "3.10.4"
  },
  "platform": {
    "release": "4.19.0-18-amd64",
    "system": "Linux"
  },
  "pyOpenSSL": {
    "openssl_version": "",
    "version": null
  },
  "requests": {
    "version": "2.27.1"
  },
  "system_ssl": {
    "version": "101010ef"
  },
  "urllib3": {
    "version": "1.26.9"
  },
  "using_charset_normalizer": true,
  "using_pyopenssl": false
}
```

You're not telling urllib3 to use that SSL context with the proxy, you'll want to look at https://github.com/psf/requests/blob/2d5517682b3b38547634d153cea43d48fbc8cdb5/requests/adapters.py#L167

Thanks, that fixed it.