bpo-37702: Fix SSL's certificate-store leak on Windows (GH-15632)

ssl_collect_certificates function in _ssl.c has a memory leak.
Calling CertOpenStore() and CertAddStoreToCollection(), a store's refcnt gets incremented by 2.
But CertCloseStore() is called only once and the refcnt leaves 1.

Misc/NEWS.d/next/Windows/2019-07-29-16-49-31.bpo-37702.Lj2f5e.rst

@@ -0,0 +1,2 @@
+Fix memory leak on Windows in creating an SSLContext object or
+running urllib.request.urlopen('https://...').

Modules/_ssl.c

@@ -5581,6 +5581,7 @@ ssl_collect_certificates(const char *store_name)
             if (result) {
                 ++storesAdded;
             }
+            CertCloseStore(hSystemStore, 0);  /* flag must be 0 */
         }
     }
     if (storesAdded == 0) {