Minor optimization for SSLObject.read call

[tarasko]

While looking at perf output I noticed that a new Long object was constructed before we call SSLObject.read
Indeed, Cython generates code to make a python object, because SSL_READ_MAX_SIZE is a constant of primitive type.
Since the value is big, it is not cached by Python, and a new object is constructed every time.

+591: if not self._sslobj_read(SSL_READ_MAX_SIZE):
__pyx_t_5 = NULL;
__Pyx_INCREF(__pyx_v_self->_sslobj_read);
__pyx_t_6 = __pyx_v_self->_sslobj_read;
__pyx_t_7 = __Pyx_PyLong_From___pyx_anon_enum(__pyx_e_6uvloop_4loop_SSL_READ_MAX_SIZE); if (unlikely(!__pyx_t_7)) __PYX_ERR(25, 591, __pyx_L3_error)
__Pyx_GOTREF(__pyx_t_7);
__pyx_t_8 = 1;