Update additional Python 3.6 versions to 3.7 (web-platform-tests#39079)

Follow-up to web-platform-tests#38911.

docs/running-tests/from-local-system.md

@@ -7,7 +7,7 @@ The tests are designed to be run from your local computer.
 Running the tests requires `python`, `pip` and `virtualenv`, as well as updating
 the system `hosts` file.
 
-WPT requires Python 3.6 or higher.
+WPT requires Python 3.7 or higher.
 
 The required setup is different depending on your operating system.
 

resources/test/tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py36,py37,py38,py39,py310
+envlist = py37,py38,py39,py310
 skipsdist=True
 
 [testenv]

wpt

@@ -2,8 +2,8 @@
 
 if __name__ == "__main__":
     import sys
-    if sys.version_info < (3, 6):
-        sys.stderr.write("wpt requires Python 3.6 or higher\n")
+    if sys.version_info < (3, 7):
+        sys.stderr.write("wpt requires Python 3.7 or higher\n")
         sys.exit(1)
 
     from tools.wpt import wpt