DRAFT Roadmap Requests 3.0

[jrfnl]

This issue is intended to contain a tentative roadmap for things to be included/removed in Requests 3.0.

For now, the most important function of this issue is to contain an action list to keep track of follow-up actions from current PRs/issues, which need to be handled in a future major release.

Action list

• Remove support for the deprecation notice silencing about use of the PSR-0 classes.
  I.e. remove support for the REQUESTS_SILENCE_PSR0_DEPRECATIONS constant.
  Related: Requests 2.0.0: introduce namespaces #519
• If possible: drop support for PHP < 7.1.
• If minimum PHP 7.1: replace input validation in methods with parameter type declarations wherever possible.
  Related: Add input validation #593
• If minimum PHP 7.1: add return type declarations wherever possible (non-union types).