Add `python/lib/pyspark.zip` to `.gitignore`. After merging #5580, `python/lib/pyspark.zip` will be generated when building Spark.

Author: zsxwing <zsxwing@gmail.com>

Closes #6017 from zsxwing/gitignore and squashes the following commits:

39b10c4 [zsxwing] Ignore python/lib/pyspark.zip