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_operator_overloading.cpp

@@ -62,6 +62,25 @@ namespace std {
     };
 }
 
+// MSVC warns about unknown pragmas, and warnings are errors.
+#ifndef _MSC_VER
+  #pragma GCC diagnostic push
+  // clang 7.0.0 and Apple LLVM 10.0.1 introduce `-Wself-assign-overloaded` to
+  // `-Wall`, which is used here for overloading (e.g. `py::self += py::self `).
+  // Here, we suppress the warning using `#pragma diagnostic`.
+  // Taken from: https://github.com/RobotLocomotion/drake/commit/aaf84b46
+  // TODO(eric): This could be resolved using a function / functor (e.g. `py::self()`).
+  #if (__APPLE__) && (__clang__)
+    #if (__clang_major__ >= 10) && (__clang_minor__ >= 0) && (__clang_patchlevel__ >= 1)
+      #pragma GCC diagnostic ignored "-Wself-assign-overloaded"
+    #endif
+  #elif (__clang__)
+    #if (__clang_major__ >= 7)
+      #pragma GCC diagnostic ignored "-Wself-assign-overloaded"
+    #endif
+  #endif
+#endif
+
 TEST_SUBMODULE(operators, m) {
 
     // test_operator_overloading
@@ -144,3 +163,7 @@ TEST_SUBMODULE(operators, m) {
         .def_readwrite("b", &NestC::b);
     m.def("get_NestC", [](const NestC &c) { return c.value; });
 }
+
+#ifndef _MSC_VER
+  #pragma GCC diagnostic pop
+#endif