Cert verify on subjectAltName->"IP Address" not working when doing IPv6 via a proxy

[derekhiggins]

Subject

Verifying a https certificate by the subjectAltName->"IP Address" fails when the accessing a server by IPv6 address through a Proxy

in the case where a proxy is used the Host IP used for comparison is wrapped in square brackets

Environment

[derekh@u07 urllib3]$ python
Python 3.6.8 (default, Aug 24 2020, 17:57:11) 
[GCC 8.3.1 20191121 (Red Hat 8.3.1-5)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import platform
>>> import urllib3
>>> 
>>> print("OS", platform.platform())
OS Linux-4.18.0-240.22.1.el8_3.x86_64-x86_64-with-centos-8.3.2011
>>> print("Python", platform.python_version())
Python 3.6.8
>>> print("urllib3", urllib3.__version__)
urllib3 1.24.2

Also reproduced on the master branch
>>> print("urllib3", urllib3.__version__)
urllib3 2.0.0.dev0

Steps to Reproduce

This works (without a proxy)

> conn = urllib3.connection_from_url('https://[fd00:1101::7d50:2883:3df9:68ce]:9999', ca_certs='ca.crt')
> response = conn.request('GET', '/')
> print(response.data)
HELLO

Accessing the same URL via a proxy doesn't

> proxy = urllib3.ProxyManager('http://[fd00:1101::1]:3128', ca_certs='ca.crt')
> conn = proxy.connection_from_url('https://[fd00:1101::7d50:2883:3df9:68ce]:9999')
> try:
>     response = conn.request('GET', '/', retries=0)
> except Exception as e:
>     print(e)
HTTPSConnectionPool(host='fd00:1101::7d50:2883:3df9:68ce', port=9999): Max retries exceeded with url: / (Caused by SSLError(CertificateError("hostname '[fd00:1101::7d50:2883:3df9:68ce]' doesn't match 'FD00:1101:0:0:7D50:2883:3DF9:68CE\\n'",),))