Remove Python 3.7 support

[mrmundt]

Fixes NA

Summary/Motivation:

Per our policy, we will drop support for Python 3.7 after the next release. This PR makes appropriate changes.

Changes proposed in this PR:

• Drop Python 3.7 support
• Deprecate / relocate pyomo.common.backports because it's no longer necessary
• Remove unnecessary checks / workarounds to Python 3.7

Legal Acknowledgement

By contributing to this software project, I have read the contribution guide and agree to the following terms and conditions for my contribution:

1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[jsiirola]

Two requested changes, and of course, the obligatory note to not merge this until after the August release.

[blnicho]

One minor comment/question on the PyPy versions listed in the README. Also, we'll need to remove the 3.7 test from Jenkins after this gets merged.