Follow-up to https://github.com/sendgrid/sendgrid-python/pull/914

This change adds information about top-level dependencies of the package to various places of its docs.

[x] I acknowledge that all my contributions will be made under the project's license
[x] I have made a material change to the repo (functionality, testing, spelling, grammar)
[x] I have read the Contribution Guidelines and my PR follows them
[x] I have titled the PR appropriately
[x] I have updated my branch with the master branch
[ ] I have added tests that prove my fix is effective or that my feature works
[x] I have added necessary documentation about the functionality in the appropriate .md file
[ ] I have added inline documentation to the code I modified

sendgrid / sendgrid-python