pytester: LineMatcher: assert collections.abc.Sequence

This can be helpful when passing a set accidentally.

changelog/4931.feature.rst

@@ -0,0 +1 @@
+pytester's ``LineMatcher`` asserts that the passed lines are a sequence.

src/_pytest/pytester.py

@@ -4,6 +4,7 @@
 from __future__ import print_function
 
 import codecs
+import collections
 import gc
 import os
 import platform
@@ -1235,6 +1236,8 @@ def _getlines(self, lines2):
             lines2 = Source(lines2)
         if isinstance(lines2, Source):
             lines2 = lines2.strip().lines
+        else:
+            assert isinstance(lines2, collections.abc.Sequence)
         return lines2
 
     def fnmatch_lines_random(self, lines2):