PyArray_DescrFromType returns a new reference, not borrowed one

Test for dtype checks now succeed without warnings

numpy: Add test for explicit dtype checks. At present, int64 + uint64 do not exactly match dtype(...).num

* Added ability to convert from Python datetime.date and datetime.time to C++ system_clock::time_point

* Protect std::min/max functions from windows.h min/max
Removed check for windows min/max

This is only necessary if `get_internals` is called for the first time in a given module when the running thread is in a GIL-released state.

Fixes #1364

* Test dtype field order in numpy dtype tests

When running tests with NumPy 1.14 or later this test exposes the
"invalid buffer descriptor" error reported in #1274.

* Create dtype_ptr with ordered fields

* Fix casting of time points with non-system-clock duration on Windows

Add explicit `time_point_cast` to time point with duration of system
clock. Fixes Visual Studio compile error.

* Add test case for custom time points casting

In def_readonly and def_readwrite, there is an assertion that the member comes
from the class or a base class:

    static_assert(std::is_base_of<C, type>::value, "...");

However, if C and type are the same type, is_base_of will still only be true
if they are the same _non-union_ type.  This means we can't define accessors
for the members of a union type because of this assertion.

Update the assertion to test

    std::is_same<C, type>::value || std::is_base_of<C, type>::value

which will allow union types, or members of base classes.

Also add a basic unit test for accessing unions.

* Fix async Python functors invoking from multiple C++ threads (#1587)

Ensure GIL is held during functor destruction.

* Add async Python callbacks test that runs in separate Python thread

In some cases the user of pythonbuf needs to allocate the internal
buffer to a specific size e.g. for performance or to enable synchronous
writes to the buffer.
By changing `pythonbuf::d_buffer` to be dynamically allocated we can now
enable these use-cases while still providing the default behavior of
allocating a 1024 byte internal buffer (through a default parameter).

fix some uninitialized members in `value_and_holder` for
some of the constructurs.

Found with coverity in a downstream project.

I found that the numpy array tests already contained an empty-shaped
array test, but none with data in it.

Following PEP 3118, scalars have an empty shape and ndim 0. This
works already and is now also documented/covered by a test.