added PGPKeyCollection to pgpy.__init__.py so it can be used directly from `import pgpy` as `pgpy.PGPKeyCollection`
added PGPKeyCollection to pgpy.__init__.py so it can be used directly from `import pgpy` as `pgpy.PGPKeyCollection`

SecurityInnovation/PGPy,J08nY/PGPy,SecurityInnovation/PGPy,J08nY/PGPy,SecurityInnovation/PGPy