cmake/modules: use exact version of python3 when finding cython

cmake/modules: use exact version of python3 when finding cython

* CMakeLists.txt:
    always pass "EXACT" to find_package(Python3).
    because per cmake document, "EXACT" only takes effect when
    <Package>_FIND_VERSION_COUNT is greater than 1, where <Package>
    is "Python3". see also cmake/modules/FindPython/Support.cmake
* cmake/modules/AddCephTest.cmake:
    drop redundant find_package(Python3) calls. since Python3 is
    a mandatory requirement for building Ceph, we only need a
    single call of find_package(Python3..) in the top of the source
    tree. the only possible case to repeat it is to ensure that we
    have the correct version of Python3 used in following CMake
    script. but there is no need to repeat it if we just want to
    ensure that we have a python3 interpretor in place.
* cmake/modules/Distutils.cmake:
    always pass "EXACT" to find_package(Python3).
    we should always pass EXACT to find_package() when finding python3,
    this is a follow-up of e2babdfae8c99f39f99a7c8a8f966299b2e62b19

Signed-off-by: Kefu Chai <tchaikov@gmail.com>