Add explicit dependency on pygments

It's used for the syntax highlighting, and prompt_toolkit no longer
depends on it.

setup.py

@@ -196,6 +196,7 b' install_requires = ['
     'simplegeneric>0.8',
     'traitlets',
     'prompt_toolkit>=0.58',
+    'pygments',
 ]
 
 # Platform-specific dependencies: