Use pycountry instead of iso-639 for languages

The latter is a fork that hasn't been updated since 2016 and the
original still seems to be well maintained, with recent database
updates as well as tests for Python 3.7.

Also, pycountry supports ISO 3166-2 (administrative zones), which
we could eventually use for sub regions.

Pipfile

@@ -12,9 +12,9 @@ flake8 = "*"
 pandas = "*"
 python-stdnum = "*"
 xlrd = "*"
-iso-639 = "*"
 requests = "*"
 requests-cache = "*"
+pycountry = "*"
 
 [requires]
 python_version = "3.7"