I encountered this error when attempting to use the AWS CLI today and at first I couldn't find any help on the Internet.
'ascii' codec can't decode byte 0xc3 in position 7: ordinal not in range(128)
A lot of the discussion I found wasn't related to AWS CLI. Some people didn't have UTF 8 enabled, and other people were having problems using with PIP on AWS micro instances and needed more RAM. The few posts that I did find where people were having the same issue were unanswered.
Comic Credit: XKCD
I was suspicious that it was an authorization issue because the help and configuration switches both worked fine, as long as it wasn't trying to pull information from AWS.
Eventually, I found bug #708 UnicodeDecodeError obfuscates authentication error on the AWS-CLI github and confirmed my suspicions.
If you are having this issue double check
~/.aws/credentials, in my case when I copied the key a non printing return or newline character had snuck into my paste which was why I got the unicode error.