Fix #113: Add Pygments>=2.1.3,<3.0.0 requirement

[donnemartin]

The problem and the fix are described here: https://github.com/awslabs/aws-shell/issues/113#issuecomment-205132744

Steps to verify fix:

• Have Pygments previously installed, say version 1.6
  • pip install Pygments==1.6
• Install aws-shell, Pygments is updated to 2.1.3
  • pip install aws-shell

aws-shell starts up without issues.

As for the medium term, note the latest prompt-toolkit 0.60 has actually removed the Pygments dependency entirely: jonathanslenders/python-prompt-toolkit@401cbbb#diff-2eeaed663bd0d25b7e608891384b7298.

When we move to prompt-toolkit>=0.60 we might need to revisit Pygments. Seems the latest prompt-toolkit has similar support built-in.

awscli==1.10.17
boto3==1.3.0
botocore==1.4.8
colorama==0.3.3
configobj==5.0.6
docutils==0.12
jmespath==0.9.0
prompt-toolkit==0.52
pyasn1==0.1.9
Pygments==2.1.3
python-dateutil==2.5.2
rsa==3.3
s3transfer==0.0.1
six==1.10.0
wcwidth==0.1.6
wheel==0.24.0

[jamesls]

:shipit: