Add Python 3.8 to considered versions in CMake for additional
hints.
https://cmake.org/cmake/help/v3.2/module/FindPythonLibs.html

* test pair-copyability on C++17 upwards

The stdlib falsely detects containers like M=std::map<T, U>
as copyable, even when one of T and U is not copyable.
Therefore we cannot rely on the stdlib dismissing std::pair<T, M>
by itself, even on C++17.

* fix is_copy_assignable

bind_map used std::is_copy_assignable which suffers from the same problems
as std::is_copy_constructible, therefore the same fix has been applied.

* created tests for copyability

Don't assume that just because the language version is C++17 that the
standard library offers all C++17 features, too.  When using clang-6.0
and --std=c++17 on Ubuntu 18.04 with libstdc++, __cpp_sized_deallocation
is false.

When building with `-Werror,-Wmissing-prototypes`, `clang` complains about missing prototypes for functions defined through macro expansions. This PR adds the missing prototypes.

```
error: no previous prototype for function 'pybind11_init_impl_embedded' [
-Werror,-Wmissing-prototypes]                                           
PYBIND11_EMBEDDED_MODULE(embedded, mod) {                                             
^                                                                           
external/pybind11/include/pybind11/embed.h:61:5: note: expanded from macro 'PYBIND11_EMBEDDED_MODULE'
    PYBIND11_EMBEDDED_MODULE_IMPL(name)                                       \
    ^                                                                         
external/pybind11/include/pybind11/embed.h:26:23: note: expanded from macro 'PYBIND11_EMBEDDED_MODULE_IMPL'
      extern "C" void pybind11_init_impl_##name() {      \                  
                      ^                                             
<scratch space>:380:1: note: expanded from here                     
pybind11_init_impl_embedded                                                                                                                      
^                                                                                                                                                
1 error generated.
```

* Adapt to python3.8 C API change

Do `Py_DECREF(type)` on all python objects on deallocation

fix #1946

* Add bare python3.8 build to CI matrix

While numpy/scipy wheels are available, run python3.8 test without them

[ci skip]

* fix: Avoid conversion to `int_` rhs argument of enum eq/ne

* test: compare unscoped enum with strings

* suppress comparison to None warning

* test unscoped enum arithmetic and comparision with unsupported type

* Make `overload_cast_impl` available in C++11 mode.

Narrow the scope of the `#if defined(PYBIND11_CPP14)` block around overload_cast to only
cover the parts where C++14 is stricly required. Thus, the implementation in
`pybind11::details::overload_cast_impl` is still available in C++11 mode.

* PR #1581: Modify test to use overload_cast_impl, update docs and change log

Dynamically resolving __contains__ on each call is wasteful since set
has a public PySet_Contains function.

The -Wmissing-prototypes Clang warning (or -Wmissing-declarations on
GCC) is very useful to avoid accidents where a function definition in a
source file doesn't match the corresponding declaration in a header
file, as it would warn already during compilation and not much later
during link time.

Unfortunately this means that exported functions defined only in the
source file (usually the ones annotated with `extern "C"`) will cause
this warning to be emitted too (on Clang, GCC has a slightly different
behavior with -Wmissing-declarations and doesn't warn here). This fixes
the warning by providing a declaration right before the definition.

Clang has a bug [1] in x86 Windows that is exposed by the use of lambdas with "unforwardable" prototypes. The error is "error: cannot compile this forwarded non-trivially copyable parameter yet", and the message was introduced in [2] (used to be an assertion).

[1] https://llvm.org/bugs/show_bug.cgi?id=28299
[2] https://github.com/microsoft/checkedc-clang/commit/feb1567e07573100ea14f9aea02f81463e791496

* In internals.h, only look at _DEBUG when compiling with MSVC.

(_DEBUG is a MSVC-specific macro.)