Minor pytest maintenance (#702)

* Add `pytest.ini` config file and set default options there instead of
  in `CMakeLists.txt` (command line arguments).

* Change all output capture from `capfd` (filedescriptors) to `capsys`
  (Python's `sys.stdout` and `sys.stderr`). This avoids capturing
  low-level C errors, e.g. from the debug build of Python.

* Set pytest minimum version to 3.0 to make it easier to use new
  features. Removed conditional use of `excinfo.match()`.

* Clean up some leftover function-level `@pytest.requires_numpy`.

tests/conftest.py

@@ -103,9 +103,9 @@ class Capture(object):
 
 
 @pytest.fixture
-def capture(capfd):
-    """Extended `capfd` with context manager and custom equality operators"""
-    return Capture(capfd)
+def capture(capsys):
+    """Extended `capsys` with context manager and custom equality operators"""
+    return Capture(capsys)
 
 
 class SanitizedString(object):