Fix #354, fix #389

Overhaul the pi_info system:

Pin factories are now capable of generating pi_info themselves (although
currently they all just look up the revision and call pi_info with a
specific one).

PiGPIOPin will now return pi_info for the remote pi which can be
specified by parameter or implicitly by the environment vars.

Overvolted Pis should work properly no matter what (some argument over
whether the revision 7 or 8 chars in this case; both should work). Added
some minor tweaks for the new camera-capable Pi Zero

Finally, added a bunch of tests for pins.data

gpiozero/pins/__init__.py

@@ -6,6 +6,9 @@ from __future__ import (
     )
 str = type('')
 
+import io
+
+from .data import pi_info
 from ..exc import (
     PinInvalidFunction,
     PinSetInput,
@@ -47,6 +50,7 @@ class Pin(object):
     * :meth:`_set_edges`
     * :meth:`_get_when_changed`
     * :meth:`_set_when_changed`
+    * :meth:`pi_info`
     * :meth:`output_with_state`
     * :meth:`input_with_pull`
 
@@ -243,3 +247,48 @@ class Pin(object):
         property will raise :exc:`PinEdgeDetectUnsupported`.
         """)
 
+    @classmethod
+    def pi_info(cls):
+        """
+        Returns a :class:`PiBoardInfo` instance representing the Pi that
+        instances of this pin class will be attached to.
+
+        If the pins represented by this class are not *directly* attached to a
+        Pi (e.g. the pin is attached to a board attached to the Pi, or the pins
+        are not on a Pi at all), this may return ``None``.
+        """
+        return None
+
+
+class LocalPin(Pin):
+    """
+    Abstract base class representing pins attached locally to a Pi. This forms
+    the base class for local-only pin interfaces (:class:`RPiGPIOPin`,
+    :class:`RPIOPin`, and :class:`NativePin`).
+    """
+    _PI_REVISION = None
+
+    @classmethod
+    def pi_info(cls):
+        """
+        Returns a :class:`PiBoardInfo` instance representing the local Pi.
+        The Pi's revision is determined by reading :file:`/proc/cpuinfo`. If
+        no valid revision is found, returns ``None``.
+        """
+        # Cache the result as we can reasonably assume it won't change during
+        # runtime (this is LocalPin after all; descendents that deal with
+        # remote Pis should inherit from Pin instead)
+        if cls._PI_REVISION is None:
+            with io.open('/proc/cpuinfo', 'r') as f:
+                for line in f:
+                    if line.startswith('Revision'):
+                        revision = line.split(':')[1].strip().lower()
+                        overvolted = revision.startswith('100')
+                        if overvolted:
+                            revision = revision[-4:]
+                        cls._PI_REVISION = revision
+                        break
+                if cls._PI_REVISION is None:
+                    return None # something weird going on
+        return pi_info(cls._PI_REVISION)
+