Make pycapnp GIL friendly

Now calling `wait` from one thread will not block all threads

capnp/includes/capnp_cpp.pxd

@@ -51,7 +51,7 @@ cdef extern from "kj/memory.h" namespace " ::kj":
     Own[PyRefCounter] makePyRefCounter" ::kj::heap< PyRefCounter >"(PyObject *)
 
 cdef extern from "kj/async.h" namespace " ::kj":
-    cdef cppclass Promise[T]:
+    cdef cppclass Promise[T] nogil:
         Promise()
         Promise(Promise)
         Promise(T)

capnp/includes/types.pxd

@@ -1,5 +1,4 @@
 from cpython.ref cimport PyObject, Py_INCREF, Py_DECREF
-from cpython.exc cimport PyErr_Clear
 from libc.stdint cimport *
 ctypedef unsigned int uint
 ctypedef uint8_t byte