Remove mixed-indentation from the codebase

ChangeLog

@@ -8,11 +8,15 @@ Release date: TBA
 
 * bad-continuation and bad-whitespace have been removed, black or another formatter can help you with this better than Pylint
 
-Close #246, #289, #638, #747, #1148, #1179, #1943, #2041, #2301, #2304, #2944, #3565
+    Close #246, #289, #638, #747, #1148, #1179, #1943, #2041, #2301, #2304, #2944, #3565
 
 * The no-space-check option has been removed. It's no longer possible to consider empty line like a `trailing-whitespace` by using clever options
 
-Close #1368
+    Close #1368
+
+* mixed-indentation has been removed, it is no longer useful since TabError is included directly in python3
+
+   Close #2984 #3573
 
 What's New in Pylint 2.5.1?
 ===========================

doc/whatsnew/2.6.rst

@@ -19,3 +19,5 @@ Other Changes
 * `bad-continuation` and `bad-whitespace` have been removed. `black` or another formatter can help you with this better than Pylint
 
 * The `no-space-check` option has been removed, it's no longer possible to consider empty line like a `trailing-whitespace` by using clever options.
+
+* `mixed-indentation` has been removed, it is no longer useful since TabError is included directly in python3

pylint/checkers/format.py

@@ -149,11 +149,6 @@
         "Used when an unexpected number of indentation's tabulations or "
         "spaces has been found.",
     ),
-    "W0312": (
-        "Found indentation with %ss instead of %ss",
-        "mixed-indentation",
-        "Used when there are some mixed tabs and spaces in a module.",
-    ),
     "W0301": (
         "Unnecessary semicolon",  # was W0106
         "unnecessary-semicolon",
@@ -744,13 +739,6 @@ def check_indent_level(self, string, expected, line_num):
             level += 1
         suppl = ""
         while string and string[0] in " \t":
-            if string[0] != indent[0]:
-                if string[0] == "\t":
-                    args = ("tab", "space")
-                else:
-                    args = ("space", "tab")
-                self.add_message("mixed-indentation", args=args, line=line_num)
-                return level
             suppl += string[0]
             string = string[1:]
         if level != expected or suppl:
@@ -762,7 +750,6 @@ def check_indent_level(self, string, expected, line_num):
                 line=line_num,
                 args=(level * unit_size + len(suppl), i_type, expected * unit_size),
             )
-        return None
 
 
 def register(linter):