Refactor get_delta_from_subject() method

Refactors the get_delta_from_subject() method for a few benefits:

* Checks type more idiomatically (using isinstance instead of type)
* Raise a TypeError if passed a type that isn't able to be handled
* Return immediately if passed a timedelta, so the rest of the method
  doesn't need to be inside an else block

ago.py

@@ -4,18 +4,27 @@ units = ("year", "day", "hour", "minute", "second", "millisecond", "microsecond"
 
 
 def get_delta_from_subject(subject):
-    subject_type = type(subject)
-    if subject_type is timedelta:
-        delta = subject
+    """Convert the subject to a timedelta and return it."""
+    if isinstance(subject, timedelta):
+        return subject
+
+    if isinstance(subject, datetime):
+        dt = subject
     else:
-        if subject_type is int or subject_type is float:
-            dt = datetime.fromtimestamp(float(subject))
-        else:
-            # assume subject is a datetime object and strip timezone data.
-            dt = subject.replace(tzinfo=None)
-        # finally, create a timedelta from datetime.
-        delta = datetime.now() - dt
-    return delta
+        # if it's not a datetime, assume it's a unix timestamp
+        try:
+            subject = float(subject)
+        except ValueError:
+            # some unknown type that couldn't be converted to a float
+            raise TypeError("Unsupported subject type")
+
+        dt = datetime.fromtimestamp(subject)
+
+    # strip timezone data
+    dt = dt.replace(tzinfo=None)
+
+    # finally, return a timedelta from the datetime
+    return datetime.now() - dt
 
 
 def delta2dict(delta):