Merge pull request #71 from cs50/eprint

adds eprint back through June 2019

Author: Kareem Zidane

setup.py

@@ -16,5 +16,5 @@
     package_dir={"": "src"},
     packages=["cs50"],
     url="https://github.com/cs50/python-cs50",
-    version="3.0.2"
+    version="3.1.0"
 )

src/cs50/cs50.py

@@ -32,6 +32,18 @@ def write(self, x):
 sys.stdout = flushfile(sys.stdout)
 
 
+def eprint(*args, **kwargs):
+    """
+    Print an error message to standard error, prefixing it with
+    file name and line number from which method was called.
+    """
+    end = kwargs.get("end", "\n")
+    sep = kwargs.get("sep", " ")
+    (filename, lineno) = inspect.stack()[1][1:3]
+    print("{}:{}: ".format(filename, lineno), end="")
+    print(*args, end=end, file=sys.stderr, sep=sep)
+
+
 def formatException(type, value, tb):
     """
     Format traceback, darkening entries from global site-packages directories