chore: add constraints file (#649)

Add constraints file to test lower bounds
diff --git a/testing/constraints-3.10.txt b/testing/constraints-3.10.txt
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/testing/constraints-3.10.txt
diff --git a/testing/constraints-3.11.txt b/testing/constraints-3.11.txt
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/testing/constraints-3.11.txt
diff --git a/testing/constraints-3.6.txt b/testing/constraints-3.6.txt
new file mode 100644
index 0000000..ff7f099
--- /dev/null
+++ b/testing/constraints-3.6.txt
@@ -0,0 +1,14 @@
+# This constraints file is used to check that lower bounds
+# are correct in setup.py
+# List *all* library dependencies and extras in this file.
+# Pin the version to the lower bound.
+#
+# e.g., if setup.py has "foo >= 1.14.0, < 2.0.0dev",
+# Then this file should have foo==1.14.0
+cachetools==2.0.0
+pyasn1-modules==0.2.1
+setuptools==40.3.0
+six==1.9.0
+rsa==4.6
+rsa==3.1.4
+aiohttp==3.6.2
\ No newline at end of file
diff --git a/testing/constraints-3.7.txt b/testing/constraints-3.7.txt
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/testing/constraints-3.7.txt
diff --git a/testing/constraints-3.8.txt b/testing/constraints-3.8.txt
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/testing/constraints-3.8.txt
diff --git a/testing/constraints-3.9.txt b/testing/constraints-3.9.txt
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/testing/constraints-3.9.txt