Urllib3 >1.26.x | Introduces default User-Agent header

With pyrequest pullrequest https://github.com/psf/requests/commit/03957eb1c2b9a1e5e6d61f5e930d7c5ed7cfe853 v2.25.0 urllib3 is updated to v1.26.0. with this version bump urllib3 has changed default behavior in regards to header "User-Agent" : https://pypi.org/project/urllib3/

urllib3 1.26.0

Added default User-Agent header to every request (Pull #1750)
Added urllib3.util.SKIP_HEADER for skipping User-Agent, Accept-Encoding, and Host headers from being automatically emitted with requests (Pull #2018)

Urllib3 now introduces default User-Agent header when no user-agent header is set. Urlib3 also introduces the function "urllib3.util.SKIP_HEADER" as a means to deactivate default user-agent header.

Expected Result

Pyrequest should adopt easy means of deactivating "User-Agent" headers by utilizing the "urllib3.util.SKIP_HEADER" function

Actual Result

Default "User-agent" header is enforced when no header= is specified.
by using headers={"User-Agent": none} no longer deactivate default "user-agent" while using headers={"User-Agent": ""} will deactivate "user-agent" header

Reproduction Steps

import requests

req = Request('POST', url=BASE_URL + endpoint, data=params).prepare()  

---> Deafult "user-agent" is enforced "User-Agent: python-urllib3/1.26.2"

req = Request('POST', url=BASE_URL + endpoint, headers={"User-Agent": None}, data=params).prepare()

---> Does not deactivate "User-Agent"

req = Request('POST', url=BASE_URL + endpoint, headers={"User-Agent": ""}, data=params).prepare()

Deactivates "User-Agent" succesfully

## System Information

    $ python -m requests.help

```

This command is only available on Requests v2.16.4 and greater. Otherwise,
please provide some basic information about your system (Python version,
operating system, &c).

Python 3.8, Docker