Unfortunate convention `io`

**What do you want to achieve, please describe.**

In [PIConGPU](https://github.com/ComputationalRadiationPhysics/picongpu/issues/4802), I've stumbled across the fact that your documentation [suggests](https://openpmd-api.readthedocs.io/en/0.15.2/usage/firstwrite.html#python) to use

```python3
import openpmd_api as io
```

which coincidentally is also the name of [a standard library module](https://docs.python.org/3/library/io.html) in the Python language.

This is an unfortunate ambiguity. We are in the process of coming up with a different convention for PIConGPU ourselves but if you feel the same about this, we would be inclined to wait for the openPMD community to come up with a less ambiguous convention to adopt. Any thoughts?