matplotlibcpp.h: get it to compile with python >= 3.8, as the C API has changed

Author: amadeus84

examples/basic.cpp

@@ -1,6 +1,6 @@
 //
-// g++ -g -Wall -o basic -I/usr/include/python3.9 basic.cpp -lpython3.9
-// g++ -g -Wall -o basic $(python-config --includes) basic.cpp $(python-config --ldflags --embed)
+// g++ -g -Wall -std=c++20 -o basic -I/usr/include/python3.9 basic.cpp -lpython3.9
+// g++ -g -Wall -std=c++20 -o basic $(python-config --includes) basic.cpp $(python-config --ldflags --embed)
 //
 
 #define _USE_MATH_DEFINES

examples/basic.png

@@ -1,5 +1,5 @@
 //
-// g++ -g -Wall -o imshow $(python-config --includes) imshow.cpp $(python-config --ldflags --embed)
+// g++ -g -Wall -std=c++20 -o imshow $(python-config --includes) imshow.cpp $(python-config --ldflags --embed)
 //
 
 #define _USE_MATH_DEFINES

examples/imshow.cpp

@@ -72,15 +72,16 @@ int main()
     plt::grid(true);
     plt::show();
 
-    // Modify some data and plot again, reusing the time array:
-    for (size_t i=0; i<n; i++) data[i]=i*i-3.0;
+    // // Modify some data and plot again, reusing the time array:
+    // for (size_t i=0; i<n; i++) data[i]=i*i-3.0;
+
+    // // plt::plot((PyListObject*) tarray, data);
+    // plt::plot(tarray, data);
+    // plt::xticks({{"rotation", "20"}});
+    // plt::title("Quadratic");
+    // plt::grid(true);
+    // plt::show();
 
-    // plt::plot((PyListObject*) tarray, data);
-    plt::plot(tarray, data);
-    plt::xticks({{"rotation", "20"}});
-    plt::title("Quadratic");
-    plt::grid(true);
-    plt::show();
 
     // Unfortunately, we have to call the DateTimeList destructor explicitly
     // for now, because the destructor needs an interpreter, which we kill