Add missing deps to `environment.yml` and `clean.py`

[gunchu]

• Add missing deps execnb and nbformat to environment.yml
• Missing include in clean.py

I was able to execute clean.py after these changes as follows:

$ conda env create -f environment.yml
...
$ conda activate fastbook
...
$ python tools/clean.py
...
<success>

Without these changes I get this error when executing clean.py (steps same as above):

Traceback (most recent call last):
  File "/Users/gunchu/Desktop/Code/fastbook/tools/clean.py", line 3, in <module>
    import nbformat
ModuleNotFoundError: No module named 'nbformat'