RequestsCookieJar appends ".local" to cookie from localhost

When I make a request to localhost (on my development machine), Requests changes the domain from "localhost" to "localhost.local" when saving the cookie. Thus, I am unable to access the cookie if I specify the domain parameter in the get method.

My app requires me to get and set cookies on the session object as a persistence mechanism; I need to persist this cookie over more than one session in the python app.

## Expected Result

I would expect the domain in `RequestCookieJar` to always match the domain I used to make the request.

## Actual Result

If I make a request to localhost, instead I get the cookie saved to the domain "localhost.local". (Note that I cannot actually make requests to "localhost.local", they fail.) This puts me in a position where I must make requests to "localhost" but use "localhost.local" to get or set the cookie.

## Reproduction Steps

```
>>> import requests
>>> ses = requests.Session()
>>> ses.get('http://localhost/')
<Response [200]>
>>> ses.cookies
<RequestsCookieJar[Cookie(version=0, name='PHPSESSID', value='ui1hr334bmcr1r85jlu8k3rq65', port=None, port_specified=False, domain='localhost.local', domain_specified=False, domain_initial_dot=False, path='/', path_specified=True, secure=False, expires=None, discard=True, comment=None, comment_url=None, rest={}, rfc2109=False)]>
>>> ses.cookies.get('PHPSESSID')
'ui1hr334bmcr1r85jlu8k3rq65'
>>> ses.cookies.get('PHPSESSID', domain='localhost')
>>> ses.cookies.get('PHPSESSID', domain='localhost.local')
'ui1hr334bmcr1r85jlu8k3rq65'
```

## System Information

    $ python -m requests.help

```
{
  "chardet": {
    "version": "3.0.4"
  },
  "cryptography": {
    "version": ""
  },
  "idna": {
    "version": "2.8"
  },
  "implementation": {
    "name": "CPython",
    "version": "3.7.3"
  },
  "platform": {
    "release": "10",
    "system": "Windows"
  },
  "pyOpenSSL": {
    "openssl_version": "",
    "version": null
  },
  "requests": {
    "version": "2.22.0"
  },
  "system_ssl": {
    "version": "101000af"
  },
  "urllib3": {
    "version": "1.25.3"
  },
  "using_pyopenssl": false
}
```

Hi,

Requests uses `cookielib.make_cookies` to fill its `RequestsCookieJar`.
In turn, `cookielib.make_cookies` uses `cookielib.eff_request_host` to extract the `erhn`.
`cookielib` conforms to [rfc2965](https://www.ietf.org/rfc/rfc2965.txt) which states:
> The term effective host name is related to host name.  If a host name
   contains no dots, the effective host name is that name with the
   string .local appended to it

Therefore, unless `cookielib` adds support and defaults its policy to [rfc6265](https://www.ietf.org/rfc/rfc6265.txt), I think this is unavoidable :(

Do you have any ideas?

What if we just added a line like this to `RequestsCookieJar.get()` and `RequestCookieJar.set()` then?

```python
if '.' not in domain:
    domain += '.local'
```

That would be a pretty minor change that I don't imagine would cause issues for existing scripts, but would allow scripts to use host names without dots.

Just as a note, I am using the above two lines in my own code where I call `session.cookies.set()` and `session.cookies.get()`. Everything works as expected with that workaround. But I still think it would be good to include in the main library. I'd be happy to submit a pull request if the team would be willing to merge it.