IOError when making https requests from executable zip archive

Python has supported running packaged zip files [since 2.6](https://bugs.python.org/issue1739468). The [zipapp](https://docs.python.org/3/library/zipapp.html) module newly introduced in python 3 is supposed to make it even easier to create executable python zip archives. As part of this, the contents of the zip archive are added to `sys.path` on execution to facilitate bundling dependencies.

Unfortunately, the way `requests` currently handles TLS CA certs is incompatible with this feature- It tries to load a certificate bundle from the `cacert.pem` file provided by the `certifi` package, but this fails in the case where the provided `certifi` installation is packaged as part of a zip archive.

## Expected Result

Making https requests using `requests` as part of a zip archive succeeds.

## Actual Result

Making https calls using a requests installation from a zip file raises an `IOError`
```
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/local/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "./zipapptest.pyz/__main__.py", line 2, in <module>
  File "./zipapptest.pyz/requests/api.py", line 72, in get
  File "./zipapptest.pyz/requests/api.py", line 58, in request
  File "./zipapptest.pyz/requests/sessions.py", line 508, in request
  File "./zipapptest.pyz/requests/sessions.py", line 618, in send
  File "./zipapptest.pyz/requests/adapters.py", line 407, in send
  File "./zipapptest.pyz/requests/adapters.py", line 226, in cert_verify
OSError: Could not find a suitable TLS CA certificate bundle, invalid path: ./zipapptest.pyz/certifi/cacert.pem
```


## Reproduction Steps

I generated a minimal zip that should execute the following code:
```python
import requests
requests.get('https://www.google.com')
```

Reproduced within a `python:3` docker container

```bash
# will generate a barebones zip archive from the contents of this folder
mkdir zipapptest

# produce a 2 line __main__.py that reproduces the bug
cat >zipapptest/__main__.py <<EOF
import requests
requests.get('https://www.google.com')
EOF

# I will try to include the requests dependency in the archive
pip install -t zipapptest requests

# generate an executable zip from the above folder
python -m zipapp --python $(which python) zipapptest

# try to run the generated archive
./zipapptest.pyz
```

## System Information

    $ python -m requests.help

```
{
  "chardet": {
    "version": "3.0.4"
  },
  "cryptography": {
    "version": ""
  },
  "idna": {
    "version": "2.6"
  },
  "implementation": {
    "name": "CPython",
    "version": "3.6.3"
  },
  "platform": {
    "release": "4.9.49-moby",
    "system": "Linux"
  },
  "pyOpenSSL": {
    "openssl_version": "",
    "version": null
  },
  "requests": {
    "version": "2.18.4"
  },
  "system_ssl": {
    "version": "1000114f"
  },
  "urllib3": {
    "version": "1.22"
  },
  "using_pyopenssl": false
}
```

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).