Fix spurious pylint errors. (#7780)

This fixes (part of) the problem identified in https://github.com/certbot/certbot/pull/7657#issuecomment-586506340.

When I tested our pylint setup on Python 3.5.9, 3.6.9, or 3.6.10, tests failed with:
```
************* Module acme.challenges
acme/acme/challenges.py:57:15: E1101: Instance of 'UnrecognizedChallenge' has no 'jobj' member (no-member)
************* Module acme.jws
acme/acme/jws.py:28:16: E1101: Class 'Signature' has no '_orig_slots' member (no-member)
```
These errors did not occur for me on Python 3.6.7 or Python 3.7+.

You also cannot run our lint setup on Python 2.7 because our pinned version of pylint's dependency `asteroid` does not support Python 2. Because of this, `pylint` is not installed in the virtual environment created by `tools/venv.py` and our [`lint` environment in tox specifies that Python 3 should be used](https://github.com/certbot/certbot/blob/fd64c8c33b2176e6569d64d30776bd5fc9fd3820/tox.ini#L132).

I tried updating pylint and its dependencies to fix the problem, but they still occur so I think adding back these disable checks on these lines again is the best fix for now.

acme/acme/challenges.py

@@ -54,7 +54,7 @@ class UnrecognizedChallenge(Challenge):
         object.__setattr__(self, "jobj", jobj)
 
     def to_partial_json(self):
-        return self.jobj
+        return self.jobj  # pylint: disable=no-member
 
     @classmethod
     def from_json(cls, jobj):

acme/acme/jws.py

@@ -25,7 +25,7 @@ class Header(jose.Header):
 
 class Signature(jose.Signature):
     """ACME-specific Signature. Uses ACME-specific Header for customer fields."""
-    __slots__ = jose.Signature._orig_slots
+    __slots__ = jose.Signature._orig_slots  # pylint: disable=no-member
 
     # TODO: decoder/encoder should accept cls? Otherwise, subclassing
     # JSONObjectWithFields is tricky...