DRAFT Roadmap Requests 4.0

[jrfnl]

This issue is intended to contain a tentative roadmap for things to be included/removed in Requests 4.0.

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 major release.

Action list

• Remove the PSR-0 BC-layer from the Autoload class.
  Related: Requests 2.0.0: introduce namespaces #519
  • Remove fallback code in the Autoload class.
  • Remove classmap and files directives from Composer autoload.
  • Remove the complete library directory.