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}

tools/metrics/histograms/extract_histograms.py

@@ -57,7 +57,13 @@ XML below will generate the following five histograms:
 
 import copy
 import datetime
-import HTMLParser
+
+try:
+  import html
+except ImportError: # For Py2 compatibility
+  import HTMLParser
+  html = HTMLParser.HTMLParser()
+
 import logging
 import re
 import xml.dom.minidom
@@ -142,7 +148,7 @@ def NormalizeString(text):
 
   # Unescape using default ASCII encoding. Unescapes any HTML escaped character
   # like " etc.
-  return HTMLParser.HTMLParser().unescape(line)
+  return html.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.assertEquals('This is a summary with & and " and \'',
-                      hists['Test.Histogram']['summary'])
+    self.assertEqual('This is a summary with & and " and \'',
+                     hists['Test.Histogram']['summary'])
 
 if __name__ == "__main__":
   logging.basicConfig(level=logging.ERROR + 1)