Support any Python 3 version for building

Do not fail to build the site when using a Python
version more recent than 3.7.

Change-Id: Ia58785658b6dac1806813d7ec24e5b388e47b393
diff --git a/Pipfile b/Pipfile
index 27b2c2a..ae37401 100644
--- a/Pipfile
+++ b/Pipfile
@@ -13,7 +13,7 @@
 tqdm = "*"
 
 [requires]
-python_version = "3.7"
+python_version = "3"
 
 [pipenv]
 allow_prereleases = true