Add open_path() Python method to permit persistent device symlinks.

Adds to the functionality provided by the open() call (which hard-codes
the opening of /dev/spidev<bus>.<chipselect> device file paths).
Because the bus number allocation may not be knowable in advance (bus
numbers are dynamically allocated by the Linux kernel), open_path()
allows the user to employ udev rules to deterministically create a
symlink to the correct hardware device at boot and/or hot-plug time.

Part of doceme#129

README.md

@@ -11,7 +11,7 @@ Usage
 ```python
 import spidev
 spi = spidev.SpiDev()
-spi.open(bus, device)
+spi.open_path(spidev_devicefile_path)
 to_send = [0x01, 0x02, 0x03]
 spi.xfer(to_send)
 ```
@@ -21,7 +21,7 @@ Settings
 ```python
 import spidev
 spi = spidev.SpiDev()
-spi.open(bus, device)
+spi.open_path("/dev/spidev0.0")
 
 # Settings (for example)
 spi.max_speed_hz = 5000
@@ -43,9 +43,16 @@ spi.mode = 0b01
 Methods
 -------
 
+    open_path(filesystem_path)
+
+Connects to the specified SPI device special file, following symbolic links if
+appropriate (see note on deterministic SPI bus numbering in the Linux kernel
+below for why this can be advantageous in some configurations).
+
     open(bus, device)
 
-Connects to the specified SPI device, opening `/dev/spidev<bus>.<device>`
+Equivalent to calling `open_path("/dev/spidev<bus>.<device>")`. n.b. **Either**
+`open_path` or `open` should be used.
 
     readbytes(n)
 

spidev_module.c

@@ -1365,6 +1365,28 @@ SpiDev_open_dev(SpiDevObject *self, char *dev_path)
 }
 
 
+PyDoc_STRVAR(SpiDev_open_path_doc,
+	"open_path(spidev_path)\n\n"
+	"Connects the object to the specified SPI device.\n"
+	"open_path(X) will open the spidev character device <X> (following symbolic links if necessary).\n");
+
+static PyObject *
+SpiDev_open_path(SpiDevObject *self, PyObject *args, PyObject *kwds)
+{
+	static char *kwlist[] = {"path", NULL};
+	PyObject *py_dev_path;
+	char *dev_path;
+	if (!PyArg_ParseTupleAndKeywords(args, kwds, "O&:open", kwlist, PyUnicode_FSConverter, &py_dev_path))
+		return NULL;
+	if (py_dev_path == NULL)
+		return NULL;
+	dev_path = PyBytes_AsString(py_dev_path);
+	if (dev_path == NULL)
+		return NULL;
+	return SpiDev_open_dev(self, dev_path);
+}
+
+
 PyDoc_STRVAR(SpiDev_open_doc,
 	"open(bus, device)\n\n"
 	"Connects the object to the specified SPI device.\n"
@@ -1441,6 +1463,8 @@ PyObject *SpiDev_exit(SpiDevObject *self, PyObject *args)
 static PyMethodDef SpiDev_methods[] = {
 	{"open", (PyCFunction)SpiDev_open, METH_VARARGS | METH_KEYWORDS,
 		SpiDev_open_doc},
+	{"open_path", (PyCFunction)SpiDev_open_path, METH_VARARGS | METH_KEYWORDS,
+		SpiDev_open_path_doc},
 	{"close", (PyCFunction)SpiDev_close, METH_NOARGS,
 		SpiDev_close_doc},
 	{"fileno", (PyCFunction)SpiDev_fileno, METH_NOARGS,