Handle invalid requirement strings when using importlib.metadata

[kemzeb]

Resolves #344.

This change also:

• Makes Distribution.requires() into a generator for performance (i.e. avoid building list[Requirement] and then iterating list[Requirement and instead do both at the same time) and so that we can handle invalid requirement exceptions in PackageDAG.from_pkgs() for each individual requirement string
• Modified conftest to avoid adding a extra comma at the end and removed some older code

[kemzeb]

Here's an example of what it looks like:

$ pipdeptree
Warning!!! Invalid requirement strings found for the following distributions:
catalyst
  Skipping "scikit-image (<0.19.0>=0.16.1) ; extra == 'all'"
------------------------------------------------------------------------
catalyst==22.4
├── accelerate [required: >=0.5.1, installed: 0.29.1]
│   ├── huggingface-hub [required: Any, installed: 0.22.2]
│   │   ├── filelock [required: Any, installed: 3.13.3]
. . .