pytest: --help broken when there's a required option set in pytest_addoption()

Creating this after chatting on #pylib. When adding command line options using pytest_addoption(), if any of the arguments are set to be required (e.g. with required=True) it breaks the ability to run --help (unless you also provide the required option). This is in contrast to how a vanilla argparse config works.

Example of this not occurring with argparse:

import argparse

parser = argparse.ArgumentParser()
parser.add_argument('--foo', required=True)
args = parser.parse_args()

--help still works without the required argument, but executing the file does not (expected behavior):

$ python vanilla.py --help
usage: vanilla.py [-h] --foo FOO

optional arguments:
  -h, --help  show this help message and exit
  --foo FOO

$ python vanilla.py
usage: vanilla.py [-h] --foo FOO
vanilla.py: error: the following arguments are required: --foo

Example of this not working as expected with pytest:

$ cat conftest.py
def pytest_addoption(parser):
    parser.addoption("--foo", required=True)

--help is broken, unless you also provide the required option:

$ pytest --help
usage: pytest [options] [file_or_dir] [file_or_dir] [...]
pytest: error: the following arguments are required: --foo

$ pytest --help --foo bar
usage: pytest [options] [file_or_dir] [file_or_dir] [...]

positional arguments:
  file_or_dir
...

My environment:

$ pip list
pip (8.1.2)
py (1.4.31)
pytest (3.0.3)
setuptools (20.10.1)

$ sw_vers
ProductName:    Mac OS X
ProductVersion: 10.10.5
BuildVersion:   14F1912

• Include a detailed description of the bug or suggestion
• pip list of the virtual environment you are using
• pytest and operating system versions
• Minimal example if possible