There is an API Client Library code sample code here. My code typically looks like below which is a slight variant on the code they provide:
from google.cloud import storage
client = storage.Client(project='<myprojectname>')
mybucket = storage.bucket.Bucket(client=client, name='mybucket')
mydatapath = 'C:\whatever\something' + '\\' #etc
blob = mybucket.blob('afile.txt')
blob.upload_from_filename(mydatapath + 'afile.txt')
In case it is of interest, another method is to run the "gsutil" command line how you have typed in your Original Post using the subprocess command, e.g.:
import subprocess
subprocess.call("gsutil cp afile.txt gs://mybucket/", shell=True)
In my view, there are pros and cons of both methods depending on what you are trying to achieve - the latter method allows multi-threading if you have many files to upload whereas the former method perhaps allows better control, specification of metadata for each file, etc.