Support for ipv6 address format

[mariodev]

Describe the bug:

Exception is thrown when python agent is trying to parse url, that contains ipv6 address.

ValueError: too many values to unpack (expected 2)
  File "elasticapm/contrib/django/middleware/__init__.py", line 162, in process_response
    "request",
  File "elasticapm/traces.py", line 333, in set_context
    data = data()
  File "elasticapm/contrib/django/middleware/__init__.py", line 160, in <lambda>
    request, capture_body=self.client.config.capture_body in ("all", "transactions")
  File "elasticapm/contrib/django/client.py", line 137, in get_data_from_request
    result["url"] = get_url_dict(url)
  File "elasticapm/utils/__init__.py", line 84, in get_url_dict
    hostname, port = netloc.split(":")

To Reproduce

Not sure how can I manually replicate it, but below is the request details that is send to our webapp.

curl \
 --compressed \
 -H "Accept: */*" \
 -H "Accept-Encoding: gzip, deflate" \
 -H "Connection: close" \
 -H "Host: [::ffff:a9fe:a9fe]:80" \
 -H "User-Agent: AWS Security Scanner" \
 -H "X-Forwarded-For: 44.224.22.196, 10.255.0.2" \
 -H "X-Forwarded-Port: 443" \
 -H "X-Forwarded-Proto: https" \
 -H "X-Real-Ip: 10.255.0.2" \
 "http://[::ffff:a9fe:a9fe]/latest/dynamic/instance-identity/document"

Expected behavior:

The url containing hostname in ipv6 address, should be parsed correctly and python agent should successfully process request.

Environment (please complete the following information)

• OS: Linux
• Python version: 3.5
• Framework and version [e.g. Django 2.1]: 2.1
• APM Server version: 6.7.1
• Agent version: 5.3.0