PyYAML<6 breaks the installation on colab
**Describe the bug**

PyYAML<6 in requirements breaks the installation on Google Colab. Removing the PyYAML<6 requirement from requirements/requirements_lightning.txt solves the problem.

**Steps/Code to reproduce bug**

Just run the first cell of https://github.com/NVIDIA/NeMo/blob/stable/tutorials/nlp/Punctuation_and_Capitalization.ipynb

**Expected behavior**

Successful installation.

**Environment overview (please complete the following information)**

 - Environment location: Google Colab
 - Method of NeMo install: pip
