Add -Wdeprecated to test suite and fix associated warnings (#1191)

This commit turns on `-Wdeprecated` in the test suite and fixes several
associated deprecation warnings that show up as a result:

- in C++17 `static constexpr` members are implicitly inline; our
  redeclaration (needed for C++11/14) is deprecated in C++17.

- various test suite classes have destructors and rely on implicit copy
  constructors, but implicit copy constructor definitions when a
  user-declared destructor is present was deprecated in C++11.

- Eigen also has various implicit copy constructors, so just disable
  `-Wdeprecated` in `eigen.h`.

tests/test_call_policies.cpp

@@ -36,6 +36,8 @@ TEST_SUBMODULE(call_policies, m) {
     class Child {
     public:
         Child() { py::print("Allocating child."); }
+        Child(const Child &) = default;
+        Child(Child &&) = default;
         ~Child() { py::print("Releasing child."); }
     };
     py::class_<Child>(m, "Child")