DCC App should exclude PySide/2 explicitly in requires

For DCC App package like Maya, Houdini, Nuke or any other that ships with their own PySide Qt bindings, should exclude PySide/2 in package `requires` explicitly for avoid conflict.

```python
# package.py
requires = [
    "!PySide",
    "!PySide2",
]
```

`PyQt4/5` is fine, they don't use it.
