Add support for python setuptools-style version requirement definitions

Converting pypi packages into rez packages currently requires a bit of munging to convert the version requirements syntax to something rez can understand, and even then, it's not always an exact translation. Considering the ubiquity of setuptools, and how common it is to use pypi packages with rez, it seems to make sense for rez to support their version dependency syntax.

Primarily, supporting comma-separated version definitions. A good example would be the incredibly popular [`requests`](https://github.com/requests/requests/blob/master/setup.py#L45) package

```
requires = [
'chardet>=3.0.2,<3.1.0',
'idna>=2.5,<2.7',
'urllib3>=1.21.1,<1.23',
'certifi>=2017.4.17'
]
```