Add missing chardet, implied by pdfminer.six?

requirements/main.txt

@@ -4,6 +4,7 @@
 cffi == 1.11.5
 img2pdf == 0.3.1
 pdfminer.six == 20181108
+chardet == 3.0.4
 pikepdf == 0.3.7
 Pillow >= 5.0.0, != 5.1.0 ; sys_platform == "darwin"
 pycparser == 2.19

setup.py

@@ -249,6 +249,7 @@ setup(
         'src/ocrmypdf/lib/compile_leptonica.py:ffibuilder'
     ],
     install_requires=[
+        'chardet >= 3.0.0',
         'cffi >= 1.9.1',          # must be a setup and install requirement
         'img2pdf >= 0.3.0, < 0.4',       # pure Python, so track HEAD closely
         'pdfminer.six == 20181108',