Speed up AppVeyor build (#1021)

The `latest` build remains as is, but all others are modified to:

* Use regular Python instead of conda. `pip install` is much faster
  than conda, but scipy isn't available. Numpy is still tested.

* Compile in debug mode instead of release.

* Skip CMake build tests. For some reason, CMake configuration is very
  slow on AppVeyor and these tests are almost entirely CMake.

The changes reduce build time to about 1/3 of the original. The `latest` 
config still covers scipy, release mode and the CMake build tests, so 
the others don't need to.
diff --git a/tests/test_smart_ptr.cpp b/tests/test_smart_ptr.cpp
index 9051b5c..dccb1e9 100644
--- a/tests/test_smart_ptr.cpp
+++ b/tests/test_smart_ptr.cpp
@@ -8,6 +8,10 @@
     BSD-style license that can be found in the LICENSE file.
 */
 
+#if defined(_MSC_VER) && _MSC_VER < 1910
+#  pragma warning(disable: 4702) // unreachable code in system header
+#endif
+
 #include "pybind11_tests.h"
 #include "object.h"