CMake: Add Python 3.8 to pybind11Tools (#1974)

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

Author: ax3l

tools/pybind11Tools.cmake

@@ -12,7 +12,7 @@ if(NOT PYBIND11_PYTHON_VERSION)
   set(PYBIND11_PYTHON_VERSION "" CACHE STRING "Python version to use for compiling modules")
 endif()
 
-set(Python_ADDITIONAL_VERSIONS 3.7 3.6 3.5 3.4)
+set(Python_ADDITIONAL_VERSIONS 3.8 3.7 3.6 3.5 3.4)
 find_package(PythonLibsNew ${PYBIND11_PYTHON_VERSION} REQUIRED)
 
 include(CheckCXXCompilerFlag)