Revert "tools/metrics/histograms/extract_histograms.py: Python 3 support"

This reverts commit 465e6173.

Reason for revert: Breaks build

Original change's description:
> tools/metrics/histograms/extract_histograms.py: Python 3 support
>
> Example error:
> Traceback (most recent call last):
>   File "../../tools/metrics/histograms/generate_expired_histograms_array.py", line 14, in <module>
>     import extract_histograms
>   File "C:\Google\chromium\src\tools\metrics\histograms\extract_histograms.py", line 60, in <module>
>     import HTMLParser
> ModuleNotFoundError: No module named 'HTMLParser'
>
> Also fixed deprecation warnings:
> C:\Google\chromium\src\tools\metrics\histograms\extract_histograms.py:150: DeprecationWarning: The unescape method is deprecated and will be removed in 3.5, use html.unescape() instead.
>   return HTMLParser().unescape(line)
> ...C:\Google\chromium\src\tools\metrics\histograms\extract_histograms_test.py:481: DeprecationWarning: Please use assertEqual instead.
>   self.assertEquals('This is a summary with & and " and \'',
>
> The changes are backwards compatible.
>
> Bug: 941669
> Change-Id: Ieb22e72b37ec26886f3ca1341e487be4f0ba3ab1
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1830657
> Auto-Submit: Raul Tambre <raul@tambre.ee>
> Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
> Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#701265}

TBR=asvitkine@chromium.org,raul@tambre.ee

Change-Id: Ie6f8b307af54bd3d0ca347efef8908d013416879
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 941669
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1830287
Commit-Queue: Jon Mann <jonmann@chromium.org>
Commit-Queue: Steven Holte <holte@chromium.org>
Auto-Submit: Jon Mann <jonmann@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#701319}

tools/metrics/histograms/extract_histograms.py

@@ -57,13 +57,7 @@ XML below will generate the following five histograms:
 
 import copy
 import datetime
-
-try:
-  import html
-except ImportError: # For Py2 compatibility
-  import HTMLParser
-  html = HTMLParser.HTMLParser()
-
+import HTMLParser
 import logging
 import re
 import xml.dom.minidom
@@ -148,7 +142,7 @@ def NormalizeString(text):
 
   # Unescape using default ASCII encoding. Unescapes any HTML escaped character
   # like " etc.
-  return html.unescape(line)
+  return HTMLParser.HTMLParser().unescape(line)
 
 
 def _NormalizeAllAttributeValues(node):

tools/metrics/histograms/extract_histograms_test.py

@@ -478,8 +478,8 @@ class ExtractHistogramsTest(unittest.TestCase):
     self.assertFalse(have_errors)
     self.assertIn('Test.Histogram', hists)
     self.assertIn('summary', hists['Test.Histogram'])
-    self.assertEqual('This is a summary with & and " and \'',
-                     hists['Test.Histogram']['summary'])
+    self.assertEquals('This is a summary with & and " and \'',
+                      hists['Test.Histogram']['summary'])
 
 if __name__ == "__main__":
   logging.basicConfig(level=logging.ERROR + 1)