diff --git a/.appveyor.yml b/.appveyor.yml
index 149a8a3dc9..85445d41a2 100644
--- a/.appveyor.yml
+++ b/.appveyor.yml
@@ -19,7 +19,7 @@ install:
     if ($env:PLATFORM -eq "x64") { $env:PYTHON = "$env:PYTHON-x64" }
     $env:PATH = "C:\Python$env:PYTHON\;C:\Python$env:PYTHON\Scripts\;$env:PATH"
     python -W ignore -m pip install --upgrade pip wheel
-    python -W ignore -m pip install pytest numpy --no-warn-script-location
+    python -W ignore -m pip install pytest numpy --no-warn-script-location pytest-timeout
 - ps: |
     Start-FileDownload 'https://gitlab.com/libeigen/eigen/-/archive/3.3.7/eigen-3.3.7.zip'
     7z x eigen-3.3.7.zip -y > $null
diff --git a/include/pybind11/iostream.h b/include/pybind11/iostream.h
index 7f34bb2010..9dee755431 100644
--- a/include/pybind11/iostream.h
+++ b/include/pybind11/iostream.h
@@ -52,9 +52,11 @@ class pythonbuf : public std::streambuf {
 
                 pywrite(line);
                 pyflush();
+
+                // Placed inside gil_scoped_aquire as a mutex to avoid a race
+                setp(pbase(), epptr());
             }
 
-            setp(pbase(), epptr());
         }
         return 0;
     }
diff --git a/setup.cfg b/setup.cfg
index 80a361df97..041e62dae6 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -63,4 +63,23 @@ ignore =
 
 [mypy]
 files = pybind11
-strict = True
+python_version = 2.7
+warn_unused_configs = True
+
+# Currently (0.800) identical to --strict
+disallow_any_generics = True
+disallow_subclassing_any = True
+disallow_untyped_calls = True
+disallow_untyped_defs = True
+disallow_incomplete_defs = True
+check_untyped_defs = True
+disallow_untyped_decorators = True
+no_implicit_optional = True
+warn_redundant_casts = True
+warn_unused_ignores = True
+warn_return_any = True
+no_implicit_reexport = True
+strict_equality = True
+
+[tool:pytest]
+timeout = 300
diff --git a/tests/requirements.txt b/tests/requirements.txt
index 0b383bf5c3..8768fc1fc7 100644
--- a/tests/requirements.txt
+++ b/tests/requirements.txt
@@ -5,5 +5,6 @@ numpy==1.19.3; (platform_python_implementation!="PyPy" or sys_platform=="linux")
 pytest==4.6.9; python_version<"3.5"
 pytest==6.1.2; python_version=="3.5"
 pytest==6.2.1; python_version>="3.6"
+pytest-timeout
 scipy==1.2.3; (platform_python_implementation!="PyPy" or sys_platform=="linux") and python_version<"3.6"
 scipy==1.5.4; (platform_python_implementation!="PyPy" or sys_platform=="linux") and python_version>="3.6" and python_version<"3.10"