MAINT: probe multiple DLL handling on Win

* Test for DLL pollution in numpy/.libs/ in Azure
CI Windows Python 3.6 builds

* emit a warning with DLL names when > 1 are present
in numpy/.libs/

azure-pipelines.yml

@@ -177,6 +177,13 @@ jobs:
           pip install $_.FullName
       }
     displayName: 'Build NumPy'
+  - bash: |
+      pushd . && cd .. && target=$(python -c "import numpy, os; print(os.path.abspath(os.path.join(os.path.dirname(numpy.__file__), '.libs')))") && popd
+      pip download -d destination --only-binary --no-deps numpy==1.14
+      cd destination && unzip numpy*.whl && cp numpy/.libs/*.dll $target
+      ls $target
+    displayName: 'Add extraneous & older DLL to numpy/.libs to probe DLL handling robustness'
+    condition: eq(variables['PYTHON_VERSION'], '3.6')
   - script: pushd . && cd .. && python -c "from ctypes import windll; windll.kernel32.SetDefaultDllDirectories(0x00000800); import numpy" && popd
     displayName: 'For gh-12667; Windows DLL resolution'
   - script: python runtests.py -n --show-build-log --mode=$(TEST_MODE) -- -rsx --junitxml=junit/test-results.xml

numpy/core/__init__.py

@@ -14,11 +14,19 @@
     # convention for storing / loading the DLL from
     # numpy/.libs/, if present
     libs_path = Path(Path(__file__).absolute().parents[1], '.libs')
+    DLL_filenames = []
     if libs_path.exists():
         for filename in libs_path.glob('*openblas*dll'):
             # NOTE: would it change behavior to load ALL
             # DLLs at this path vs. the name restriction?
             WinDLL(str(filename.absolute()))
+            DLL_filenames.append(str(filename))
+    if len(DLL_filenames) > 1:
+        import warnings
+        warnings.warn("loaded more than 1 DLL from .libs:",
+                      stacklevel=1)
+        for DLL_filename in DLL_filenames:
+            warnings.warn(DLL_filename, stacklevel=1)
 
 # disables OpenBLAS affinity setting of the main thread that limits
 # python threads or processes to one core