MAINT: Add python 3.6 support to suppress_warnings

Python 3.6 has two methods for showing warnings, so we have to support
both ways (strictly speaking, on python 3.6 itself only the new would
probably be needed).

Also adds the _filters_mutated() call to replace manual registry
clearing. This is basically the fix that newer python versions have
to avoid the issue making the manual clearing unnecessary.

Closes numpygh-8176

Author: seberg

numpy/testing/utils.py

@@ -2027,6 +2027,11 @@ def __init__(self, forwarding_rule="always"):
         self._forwarding_rule = forwarding_rule
 
     def _clear_registries(self):
+        if hasattr(warnings, "_filters_mutated"):
+            # clearing the registry should not be necessary on new pythons,
+            # instead the filters should be mutated.
+            warnings._filters_mutated()
+            return
         # Simply clear the registry, this should normally be harmless,
         # note that on new pythons it would be invalidated anyway.
         for module in self._tmp_modules:
@@ -2116,6 +2121,8 @@ def __enter__(self):
             raise RuntimeError("cannot enter suppress_warnings twice.")
 
         self._orig_show = warnings.showwarning
+        if hasattr(warnings, "_showwarnmsg"):
+            self._orig_showmsg = warnings._showwarnmsg
         self._filters = warnings.filters
         warnings.filters = self._filters[:]
 
@@ -2139,20 +2146,29 @@ def __enter__(self):
                     module=module_regex)
                 self._tmp_modules.add(mod)
         warnings.showwarning = self._showwarning
+        if hasattr(warnings, "_showwarnmsg"):
+            warnings._showwarnmsg = self._showwarnmsg
         self._clear_registries()
 
         return self
 
     def __exit__(self, *exc_info):
         warnings.showwarning = self._orig_show
+        if hasattr(warnings, "_showwarnmsg"):
+            warnings._showwarnmsg = self._orig_showmsg
         warnings.filters = self._filters
         self._clear_registries()
         self._entered = False
         del self._orig_show
         del self._filters
 
+    def _showwarnmsg(self, msg):
+        self._showwarning(msg.message, msg.category, msg.filename, msg.lineno,
+                          msg.file, msg.line, use_warnmsg=msg)
+
     def _showwarning(self, message, category, filename, lineno,
                      *args, **kwargs):
+        use_warnmsg = kwargs.pop("use_warnmsg", None)
         for cat, _, pattern, mod, rec in (
                 self._suppressions + self._tmp_suppressions)[::-1]:
             if (issubclass(category, cat) and
@@ -2179,8 +2195,11 @@ def _showwarning(self, message, category, filename, lineno,
         # There is no filter in place, so pass to the outside handler
         # unless we should only pass it once
         if self._forwarding_rule == "always":
-            self._orig_show(message, category, filename, lineno,
-                            *args, **kwargs)
+            if use_warnmsg is None:
+                self._orig_show(message, category, filename, lineno,
+                                *args, **kwargs)
+            else:
+                self._orig_showmsg(use_warnmsg)
             return
 
         if self._forwarding_rule == "once":
@@ -2193,7 +2212,11 @@ def _showwarning(self, message, category, filename, lineno,
         if signature in self._forwarded:
             return
         self._forwarded.add(signature)
-        self._orig_show(message, category, filename, lineno, *args, **kwargs)
+        if use_warnmsg is None:
+            self._orig_show(message, category, filename, lineno, *args,
+                            **kwargs)
+        else:
+            self._orig_showmsg(use_warnmsg)
 
     def __call__(self, func):
         """