fix: MSVC 2017 C++17 on Python 3 regression (#3407)

* fix: MSVC 2017 C++17 on Python 3 regression

* ci: add 3.7 job on CI

Author: henryiii

.github/workflows/ci.yml

@@ -851,6 +851,10 @@ jobs:
             std: 17
             args: >
               -DCMAKE_CXX_FLAGS="/permissive- /EHsc /GR"
+          - python: 3.7
+            std: 17
+            args: >
+              -DCMAKE_CXX_FLAGS="/permissive- /EHsc /GR"
 
     steps:
     - uses: actions/checkout@v2

include/pybind11/functional.h

@@ -69,7 +69,7 @@ struct type_caster<std::function<Return(Args...)>> {
         // ensure GIL is held during functor destruction
         struct func_handle {
             function f;
-#if !(defined(_MSC_VER) && _MSC_VER == 1916 && defined(PYBIND11_CPP17) && PY_MAJOR_VERSION < 3)
+#if !(defined(_MSC_VER) && _MSC_VER == 1916 && defined(PYBIND11_CPP17))
             // This triggers a syntax error under very special conditions (very weird indeed).
             explicit
 #endif