Remove io.ascii keyword arguments that were deprecated in v6.0 #15536

What is the problem this feature will solve?

#14914 marked a number of keyword arguments as deprecated. In v7.0 these should be removed. This includes:

read():

• Reader
• Inputter
• Outputter

get_reader():

• Reader
• Inputter
• Outputter

write():

• Writer

get_writer():

• Writer

Describe the desired outcome

Providing those keywords will raise an exception instead of a DeprecationWarning.