setup: tighten version constraint on Jinja2.

I remember thinking that not constraining it properly might bite us
someday, but assumed that Jinja2 will always stay version 2. Now it
looks like Jinja2 3.0.0a1 got released, pip picks it by default, and
it's currently broken (something about markupsafe missing).

Do what needed to be done in the first place.

setup.py

@@ -27,7 +27,7 @@ setup(
     install_requires=[
         "setuptools",
         "pyvcd~=0.1.4", # for nmigen.pysim
-        "Jinja2", # for nmigen.build
+        "Jinja2~=2.11", # for nmigen.build
     ],
     packages=find_packages(),
     entry_points={