Refactor _util._is_file to handle detached sigs and data on stdin.

When gnupg.GPG.verify_file() was changed recently to take the filename
of the signature file as an argument to GnuPG, and then take the data
file to be verified on stdin, the parser in _parsers._check_options()
would call _util._is_file(), which would return False and log an error
that "'-' is not a file!". This fixes that issue by catching OSError and
treating it differently.

Additionally, I renamed the _util._is_file() parameter
`input`→`filename` to avoid overriding a builtin method.

I also replaced the `assert` lines with explicit checks, because
`assert`s are stripped out when the Python interpreter is run with -OO.

 * FIXES an issue with verification of detached signatures whose
   datafiles are read from stdin.

gnupg/_util.py

@@ -303,22 +303,31 @@ def _has_readwrite(path):
     """
     return os.access(path, os.R_OK ^ os.W_OK)
 
-def _is_file(input):
+def _is_file(filename):
     """Check that the size of the thing which is supposed to be a filename has
     size greater than zero, without following symbolic links or using
     :func:os.path.isfile.
 
-    :param input: An object to check.
+    :param filename: An object to check.
     :rtype: bool
-    :returns: True if :param:input is file-like, False otherwise.
+    :returns: True if **filename** is file-like, False otherwise.
     """
     try:
-        assert os.lstat(input).st_size > 0, "not a file: %s" % input
-    except (AssertionError, TypeError, IOError, OSError) as err:
-        log.error(err.message, exc_info=1)
-        return False
+        statinfo = os.lstat(filename)
+        log.debug("lstat(%r) with type=%s gave us %r"
+                  % (repr(filename), type(filename), repr(statinfo)))
+        if not (statinfo.st_size > 0):
+            raise ValueError("'%s' appears to be an empty file!" % filename)
+    except OSError as oserr:
+        log.error(oserr)
+        if filename == '-':
+            log.debug("Got '-' for filename, assuming sys.stdin...")
+            return True
+    except (ValueError, TypeError, IOError) as err:
+        log.error(err)
     else:
         return True
+    return False
 
 def _is_stream(input):
     """Check that the input is a byte stream.