Install pynng in Windows with VS 2019

git clone https://github.com/codypiersall/pynng
cd pynng
pip3 install -e .

def build_libs():
    """
    Builds the nng and mbedtls libs.

    """
    # The user has to have the correct Visual Studio version on the path or the
    # build will fail, possibly in exciting and mysterious ways.
    major, minor, *_ = sys.version_info

    flags = ['-DCMAKE_POSITION_INDEPENDENT_CODE:BOOL=true']
    is_64bit = sys.maxsize > 2**32
    if WINDOWS:
        if is_64bit:
            flags += ['-G', 'Visual Studio 16 2019']
            flags += ['-A', 'x64']
        else:
            flags += ['-A', 'win32']

    # if shutil.which('ninja'):
    #     # the ninja build generator is a million times faster.
    #     flags += ['-G', 'Ninja']
    build_mbedtls(flags)
    build_nng(flags)

def build_mbedtls(cmake_args):
    """
    Clone mbedtls and build it with cmake.

    """
    do = check_call
    if not os.path.exists('mbedtls'):
        do('git clone --recursive {}'.format(MBEDTLS_REPO), shell=True)
        # for local hacking, just copy a directory (network connection is slow)
        # do('cp -r ../mbedtls mbedtls', shell=True)
        # do('git checkout {}'.format(MBEDTLS_REV), shell=True, cwd='mbedtls')

    cwd = 'mbedtls/build'
    os.mkdir(cwd)
    cmake_cmd = ['cmake'] + cmake_args
    cmake_cmd += [
        '-DENABLE_PROGRAMS=OFF',
        '-DCMAKE_BUILD_TYPE=Release',
        '-DCMAKE_INSTALL_PREFIX=../prefix',
        '..'
    ]
    print('building mbedtls with:', cmake_cmd)
    do(cmake_cmd, cwd=cwd)
    do(
        'cmake --build . --config Release --target install',
        shell=True,
        cwd=cwd,
    )