diff --git a/mitogen/parent.py b/mitogen/parent.py
index fa3092c19..f301a42c5 100644
--- a/mitogen/parent.py
+++ b/mitogen/parent.py
@@ -1429,7 +1429,9 @@ def _first_stage():
             os.environ['ARGV0']=sys.executable
             os.execl(sys.executable,sys.executable+'(mitogen:CONTEXT_NAME)')
         os.write(1,'MITO000\n'.encode())
-        C=zlib.decompress(os.fdopen(0,'rb').read(PREAMBLE_COMPRESSED_LEN))
+        fp=os.fdopen(0,'rb')
+        C=zlib.decompress(fp.read(PREAMBLE_COMPRESSED_LEN))
+        fp.close()
         fp=os.fdopen(W,'wb',0)
         fp.write(C)
         fp.close()
diff --git a/tests/data/fd_check.py b/tests/data/fd_check.py
index 0a87a95e9..f3684443a 100755
--- a/tests/data/fd_check.py
+++ b/tests/data/fd_check.py
@@ -26,6 +26,7 @@ def controlling_tty():
         return None
 
 
+out_path = sys.argv[1]
 fd = int(sys.argv[2])
 st = os.fstat(fd)
 
@@ -35,7 +36,7 @@ def controlling_tty():
 else:
     buf = os.read(fd, 4).decode()
 
-open(sys.argv[1], 'w').write(repr({
+output = repr({
     'buf': buf,
     'flags': fcntl.fcntl(fd, fcntl.F_GETFL),
     'st_mode': st.st_mode,
@@ -43,4 +44,21 @@ def controlling_tty():
     'st_ino': st.st_ino,
     'ttyname': ttyname(fd),
     'controlling_tty': controlling_tty(),
-}))
+})
+
+try:
+    out_f = open(out_path, 'w')
+except Exception:
+    exc = sys.exc_info()[1]
+    sys.stderr.write("Failed to open %r: %r" % (out_path, exc))
+    sys.exit(1)
+
+try:
+    out_f.write(output)
+except Exception:
+    out_f.close()
+    exc = sys.exc_info()[1]
+    sys.stderr.write("Failed to write to %r: %r" % (out_path, exc))
+    sys.exit(2)
+
+out_f.close()
diff --git a/tox.ini b/tox.ini
index 7f0123745..0446a1810 100644
--- a/tox.ini
+++ b/tox.ini
@@ -105,8 +105,6 @@ setenv =
     NOCOVERAGE_ERASE = 1
     NOCOVERAGE_REPORT = 1
     PIP_CONSTRAINT={toxinidir}/tests/constraints.txt
-    # Print a warning on the first occurence at each module:linenno. Available Python 2.7, 3.2+.
-    PYTHONWARNINGS=default
     # Ansible 6 - 8 (ansible-core 2.13 - 2.15) require Python 2.7 or >= 3.5 on targets
     ansible6: MITOGEN_TEST_DISTRO_SPECS=centos7 centos8 debian9 debian10 debian11 ubuntu1604 ubuntu1804 ubuntu2004
     ansible7: MITOGEN_TEST_DISTRO_SPECS=centos7 centos8 debian9 debian10 debian11 ubuntu1604 ubuntu1804 ubuntu2004
@@ -137,6 +135,9 @@ setenv =
     mode_debops_common: MODE=debops_common
     mode_localhost: ANSIBLE_SKIP_TAGS=issue_776,resource_intensive
     mode_mitogen: MODE=mitogen
+    # Print a warning on the first occurence at each module:linenno.
+    # Availability: Python 2.7, 3.2+.
+    py313: PYTHONWARNINGS=default
     strategy_linear: ANSIBLE_STRATEGY=linear
 allowlist_externals =
     # Added: Tox 3.18: Tox 4.0+