Update python libjuju from 2.9.10 to 3.0.0

Change-Id: Ifdbc1cd385a7779dc0b7d24fbd803559b8c3c878
Signed-off-by: calvinosanc1 <guillermo.calvino@canonical.com>
diff --git a/requirements-test.in b/requirements-test.in
index 08df82d..1b33834 100644
--- a/requirements-test.in
+++ b/requirements-test.in
@@ -14,7 +14,7 @@
 
 asynctest
 coverage
-flake8
+flake8<5.0.0
 mock
 nose2
 requests-mock