Fixed formatting and added explicit to ctors

Author: vfdev-5

tests/custom_exceptions.py

@@ -7,5 +7,4 @@ def __init__(self, message):
         super().__init__(message)
 
     def __str__(self):
-        s = "[PythonMyException7]: " + self.message.a
-        return s
+        return "[PythonMyException7]: " + self.message.a

tests/test_exceptions.cpp

@@ -112,12 +112,12 @@ struct PythonAlreadySetInDestructor {
 };
 
 struct CustomData {
-    CustomData(const std::string &a) : a(a) {}
+    explicit CustomData(const std::string &a) : a(a) {}
     std::string a;
 };
 
 struct MyException7 {
-    MyException7(const CustomData &message) : message(message) {}
+    explicit MyException7(const CustomData &message) : message(message) {}
     CustomData message;
 };