Always line-buffer stderr for consistency across Python versions

The change in default from block-buffering in ≤3.8 to line-buffering in
≥3.9¹ made a Cram test output vary between Python versions (and thus
fail).  I could fix the Cram test various ways, but always
line-buffering stderr makes sense because we're exclusively using it for
messaging/logging.

¹ <https://docs.python.org/3/whatsnew/3.9.html#sys>

augur/__main__.py

@@ -14,11 +14,21 @@ def main():
 
         # Explicitly enable universal newlines mode so we do the right thing.
         newline=None,
+
+        # By default, stdout is line-buffered when interactive (e.g. for
+        # consistent stdio interleaving) but block-buffered when not for (I
+        # assume) better performance.
     )
     # Apply the above to stderr as well.
     sys.stderr.reconfigure(
         errors="backslashreplace",
         newline=None,
+
+        # Always line-buffer stderr since we only use it for messaging, not
+        # data output.  This is the Python default from 3.9 onwards, but we
+        # also run on 3.8 where it's not.  Be consistent regardless of Python
+        # version.
+        line_buffering=True,
     )
 
     return augur.run( argv[1:] )