Update Windows Requirements & CI

- Merge windows requirements into requirements.txt and requirements-extra.txt
- Update tox.ini to always use build.py
- Update build.py to have module only option
- Update tox.ini to text python 3.9
- Update .travis.yml to test 3.8 and 3.9 on newer Ubuntu LTS
-Update .travis.yml to work with changes to windows tox
(also update windows to 3.8)

build.py

@@ -54,6 +54,12 @@ def parse_args():
         dest="normpo",
         help="Normalize all PO files (do this before commit).",
     )
+    parser.add_option(
+        "--modules",
+        action="store_true",
+        dest="modules",
+        help="Build the python modules.",
+    )
     (options, args) = parser.parse_args()
     return options
 
@@ -182,6 +188,8 @@ def main():
         build_mergepot()
     elif options.normpo:
         build_normpo()
+    elif options.modules:
+        build_pe_modules()
     else:
         build_normal()