allow pytest to process deprecations in docstrings

We monkey-patch warnings just as we do in the original doctesting
framework.
Needs more testing.

src/conftest.py

@@ -29,7 +29,10 @@
 # - inject it into globals namespace for doctests
 import sage.all
 from sage.doctest.parsing import SageDocTestParser, SageOutputChecker
+from sage.doctest.forker import showwarning_with_traceback
+import warnings
 
+warnings.showwarning = showwarning_with_traceback
 
 class SageDoctestModule(DoctestModule):
     """

src/sage/doctest/forker.py

@@ -265,7 +265,7 @@ def showwarning_with_traceback(message, category, filename, lineno, file=None, l
     lines.extend(traceback.format_exception_only(category, category(message)))
 
     if file is None:
-        file = sys.stderr
+        file = sys.stdout
     try:
         file.writelines(lines)
         file.flush()

src/tox.ini

@@ -232,7 +232,7 @@ exclude =
 [pytest]
 python_files = *_test.py
 norecursedirs = local prefix venv build pkgs .git src/doc src/bin
-addopts = --import-mode importlib
+addopts = --import-mode importlib -p no:warnings
 doctest_optionflags = NORMALIZE_WHITESPACE ELLIPSIS
 
 [coverage:run]