Ensure chardet is installed when testing using tox

Beautiful Soup uses chardet, if installed, to detect character
encodings. This can lead to different test results based on whether
chardet is installed or not.

Requests < 2.26.0 requires chardet, but since 2.26.0 Requests requires
charset_normalizer.

Explicitly installing chardet maintains consistent test results.

tests/test_parser.py

@@ -195,5 +195,8 @@ class TestParser(unittest.TestCase):
         self.encoding_test(html, "ascii")
 
     def encoding_test(self, html, expected):
+        # If chardet is installed Beautiful Soup uses it for encoding detection.
+        # Results for html without a valid charset may differ
+        # based on chardet availability.
         soup = htmlsoup.make_soup(html)
         self.assertEqual(soup.original_encoding, expected)

tox.ini

@@ -3,6 +3,7 @@ envlist = py36, py37, py38, py39
 
 [base]
 deps =
+    chardet
     pyftpdlib
     parameterized
     pdfminer