Remove the hard dependency on pyspark. Instead make it a dev dependency, and give people the option to install it as an optional dependency.
By deafult, `pip install typedspark` will not install `pyspark`, since many platforms (e.g. Databricks) come with ``pyspark`` preinstalled. If you want to install `typedspark` with `pyspark`, you can run `pip install typedspark[pyspark]`