content-type: text/csv does not apply UTF-8-decoding by default (RFC 7111 violation?)

Created a test/repro/MWE sheet here with a cell containing the letter `ö`: https://docs.google.com/spreadsheets/d/1q02F0AjDfCo_XlgFtT7HHU96emOcyf0PRQGKGoQTXCE/edit?usp=sharing

When you HTTP GET `https://docs.google.com/spreadsheets/d/1q02F0AjDfCo_XlgFtT7HHU96emOcyf0PRQGKGoQTXCE/export?format=csv` Google generates and sends a CSV document with UTF-8-encoded text in the response body.

Fetching this URL with `requests` and accessing the response content with the `text` attribute reveals that `requests` does _not_ UTF-8-decode the response body bytes:

```text
$ python -c 'import requests; r=requests.get("https://docs.google.com/spreadsheets/d/1q02F0AjDfCo_XlgFtT7HHU96emOcyf0PRQGKGoQTXCE/export?format=csv"); print(r.text)'
foo,bar,umlaut: Ã¶Ã¶
```

Library version:
```
$ pip list | grep requests
requests                 2.25.1
```



Response headers (from a `curl -v ...`):

```
> GET /<snip> HTTP/2
> Host: doc-14-2s-sheets.googleusercontent.com
> user-agent: curl/7.69.1
> accept: */*
> 
{ [5 bytes data]
* TLSv1.3 (IN), TLS handshake, Newsession Ticket (4):
{ [264 bytes data]
* TLSv1.3 (IN), TLS handshake, Newsession Ticket (4):
{ [264 bytes data]
* old SSL session ID is stale, removing
{ [5 bytes data]
* Connection state changed (MAX_CONCURRENT_STREAMS == 100)!
} [5 bytes data]
< HTTP/2 200 
< content-type: text/csv
< x-robots-tag: noindex, nofollow, nosnippet
< cache-control: no-cache, no-store, max-age=0, must-revalidate
< pragma: no-cache
< expires: Mon, 01 Jan 1990 00:00:00 GMT
< date: Wed, 10 Feb 2021 10:12:52 GMT
< content-disposition: attachment; filename="<snip>.csv"; filename*=UTF-8''<snip>.csv
< access-control-allow-origin: *
< access-control-expose-headers: Cache-Control,Content-Disposition,Content-Encoding,Content-Length,Content-Type,Date,Expires,Pragma,Server,Transfer-Encoding
< content-security-policy: base-uri 'self';object-src 'self';report-uri https://docs.google.com/spreadsheets/cspreport;script-src 'nonce-+Qyt<snip>w' 'unsafe-inline' 'strict-dynamic' https: http: 'unsafe-eval';worker-src 'self'
< content-security-policy: frame-ancestors 'self' https://docs.google.com
< x-frame-options: ALLOW-FROM https://docs.google.com
< x-content-type-options: nosniff
< x-xss-protection: 1; mode=block
< server: GSE
< alt-svc: h3-29=":443"; ma=2592000,h3-T051=":443"; ma=2592000,h3-Q050=":443"; ma=2592000,h3-Q046=":443"; ma=2592000,h3-Q043=":443"; ma=2592000,quic=":443"; ma=2592000; v="46,43"
< accept-ranges: none
< vary: Accept-Encoding
```

So, there is `content-type: text/csv`, i.e. no `'charset'` specification.

I am not sure if [RFC 7111](https://tools.ietf.org/html/rfc7111) is the most recent / authoritative reference, but it says about the `text/csv` media type that

```
      The "charset" parameter specifies the charset employed by the CSV
      content.  In accordance with RFC 6657 [RFC6657], the charset
      parameter SHOULD be used, and if it is not present, UTF-8 SHOULD
      be assumed as the default (this implies that US-ASCII CSV will
      work, even when not specifying the "charset" parameter).  Any
      charset defined by IANA for the "text" tree may be used in
      conjunction with the "charset" parameter.
```




Given this a quick stab at https://github.com/psf/requests/pull/5747. Would appreciate an initial review and opinions. Thanks!