CI fixes (#1744)

* Fix warning that not including a cmake source or build dir will be a fatal error (it is now on newest CMakes)
    * Fixes appveyor
* Travis uses CMake 3.9 for more than a year now
* Travis dropped sudo: false in December
* Dropping Sphinx 2
- clang7: Suppress self-assign warnings; fix missing virtual dtors
- pypy:
  - Keep old version (newer stuff breaks)
  - Pin packages to extra index for speed
- travis:
  - Make docker explicit; remove docker if not needed
  - Make commands more verbose (for debugging / repro)
  - Make Ubuntu dist explicit per job
- Fix Windows
- Add names to travis

tests/test_virtual_functions.cpp

@@ -129,6 +129,7 @@ private:
 
 class NCVirt {
 public:
+    virtual ~NCVirt() { }
     virtual NonCopyable get_noncopyable(int a, int b) { return NonCopyable(a, b); }
     virtual Movable get_movable(int a, int b) = 0;