urllib3 v1.25 support

Support for `urllib3` [v1.25](https://github.com/urllib3/urllib3/releases/tag/1.25) (released today/yesterday). The usual yada-yada "my CI is broken".

Multiple packages that we depend on (eg `google.cloud.storage`, `azure.storage`) have strict version-check on `requests` using `pkg_resources.require('requests >= 2.18.0')`. Any unmet dependencies in the chain below and including `requests` will cause the check to fail and raise an exception. `urllib3<1.25` is currently one such dependency.

## Expected Result
No exception raised on import

## Actual Result
```
pkg_resources.ContextualVersionConflict: (urllib3 1.25 (/usr/local/lib/python3.6/dist-packages), Requirement.parse('urllib3<1.25,>=1.21.1'), {'requests'})

The above exception was the direct cause of the following exception:
ImportError: ``requests >= 2.18.0`` is required by the ``google.resumable_media.requests`` subpackage.
It can be installed via
    pip install google-resumable-media[requests].
```

## Reproduction Steps
```bash
pip3 install sentry-sdk google-cloud-storage
```
```python
>>> from google.cloud import storage
```

## System Information
```
/home/laurie/env-tmp/lib/python3.6/site-packages/requests/__init__.py:91: RequestsDependencyWarning: urllib3 (1.25) or chardet (3.0.4) doesn't match a supported version!
  RequestsDependencyWarning)
{
  "chardet": {
    "version": "3.0.4"
  },
  "cryptography": {
    "version": ""
  },
  "idna": {
    "version": "2.8"
  },
  "implementation": {
    "name": "CPython",
    "version": "3.6.7"
  },
  "platform": {
    "release": "4.18.0-17-generic",
    "system": "Linux"
  },
  "pyOpenSSL": {
    "openssl_version": "",
    "version": null
  },
  "requests": {
    "version": "2.21.0"
  },
  "system_ssl": {
    "version": "1010100f"
  },
  "urllib3": {
    "version": "1.25"
  },
  "using_pyopenssl": false
}
```

Note that a pull-request fixing this should close #5065 and #4961 

Also note that a solution to the above reproduction example is:
```bash
pip3 install sentry-sdk google-cloud-storage 'urllib3<1.25'
```

There’s already a PR in the works: https://github.com/kennethreitz/requests/pull/5063

To those watching, as [mentioned](https://github.com/kennethreitz/requests/pull/5063#issuecomment-487436504), we're [blocked](https://github.com/kennethreitz/requests/issues/5065#issuecomment-487423564) on `urllib3` v1.25.2

@nateprewitt FYI 1.25.2 will be released very soon, I would recommend in addition to disallowing 1.25.0 also disallow 1.25.1?

Our CI pipeline starts to get an error `[integration_py3_docker_metric] 19-04-29:10:26:57 ERROR [clipper_metric_docker.py:127] Failed to parse: http://localhost:44328/api/v1/series?match[]=clipper_mc_parse_time_ms_sum` from April 22nd, from when `urllib3` version 1.25 was released. Do you think this is related to this issue? 

@rkooo567, have you tried running the CI with the additional requirement `urllib3<1.25`?

@EpicWink Yes. And it seems like it resolves an issue. I also found out `urllib3==1.25.2` also resolves the problem. 

urllib3 1.24.3 has been released which fixes only the CRLF injection issue for 1.24.X users.

Here's a possibly dumb question: why does requests restrict urllib3 to be < 1.25? Since many other pip dependencies are going to continue to increase their minimum allowed urllib3 version, it seems like this is going to guarantee two things:
1. Every minor version bump in urllib3 will break many projects which depend on requests (e.g. anyone who is using the major Google or Microsoft Azure packages, among many others).
2. Every minor version bump in urllib3 will then require a new release of requests to allow the new version of urllib3.

Does urllib3 make breaking changes that often that we're worried about just trusting their versioning? Or is there another concern?

@plowman https://github.com/kennethreitz/requests/issues/5080 Looks like [the update to version 1.25 would make requests start up much slower](https://github.com/urllib3/urllib3/issues/1590#issue-438508993). 

Based on the number of references to this issue in other projects’ trackers, I’m sure there are a lot of other project maintainers that would appreciate a resolution to this sooner rather than later… :) (Pointing it out since issue references don’t trigger e‐mail notifications.)

+1 because it aggravates problems caused by [Pip's lack of a dependency resolver](https://github.com/pypa/pip/issues/988) in some cases: https://github.com/pradyunsg/zazo/issues/14


If you think that's fun, look at all the things that use requests and can't update urllib3.  (we do some manual checking)

http://logs.openstack.org/36/658636/3/check/requirements-tox-py27-check-uc/48d2334/job-output.txt.gz#_2019-05-14_06_33_55_887702

Resolved with #5092, release is up on pypi now. Thanks again @EpicWink