Aws-cli: Update rsa requirements

[limburgher]

Confirm by changing [ ] to [x] below to ensure that it's a bug:

• [ x] I've gone though the User Guide and the API reference
• [x ] I've searched for previous similar issues and didn't find any solution

Describe the bug
requires rsa <=3.5.0, and 4.6 is now released.

Initial testing with awscli 1.18.95 and rsa 4.6 shows no regressions.

[jekriske-lilly]

+1 on this since https://github.com/sybrenstuvel/python-rsa/blob/master/CHANGELOG.md

Regarding 4.6

No functional changes compared to version 4.2.

Whereas 4.5 is virtually retagged 4.0

[kdaily]

Hi @limburgher and @jekriske-lilly ,

See this PR for a recent change to the RSA requirements: https://github.com/aws/aws-cli/pull/5355

Now at a max of <=4.5.0 depending on the Python version.

[limburgher]

Thank you. I'm still modifying this on Fedora to work with 4.7.

[jekriske-lilly]

@kdaily Thanks, looking for 4.6 to align with the currently released version on PyPi for which the author claims would still be compatible.

[surfaceowl]

additionally, there are multiple known security issues with older versions of rsa

These have been fixed in rsa versions >=4.2.0 ...and as noted above current version is 4.6.0 (released in June 2020) -- would be great if aws would upgrade this rsa dependency soon because we have pinned the rsa version==4.6.0

Importantly aws-cli==1.18.115 fails to pip install using the new pip dependency resolver currently in beta in pip==20.2.1 (python -m pip install -r requirements.txt --use-feature=2020-resolver). This will fail all the time when pip releases 20.3