Bump protobuf requirement

py-macaroon-bakery currently requires an old protobuf version,
which is incompatible with other projects such as
python-mysql-connector [1] or the Openstack global requirements [2].

Since the project is compatible with recent protobuf versions
(4.24.x), we'll update the protobuf dependency to >=3.4,<5.0.

[1] juju/python-libjuju#914
[2] https://github.com/openstack/requirements/blob/e54dc19f590bd0ea74fd2d7ada3d0d1b3321260b/upper-constraints.txt#L377C1-L377C18

requirements.txt

@@ -5,5 +5,5 @@ requests>=2.18.4,<3.0
 PyNaCl>=1.1.2,<2.0
 pymacaroons>=0.12.0,<1.0
 six>=1.11.0,<2.0
-protobuf>=3.4.0,<4.0
+protobuf>=3.4.0,<5.0
 pyRFC3339>=1.0,<2.0