Fix broken CI and PyMySQL support

This is a combination of two patches that depend on 
each other to fix a breakage with PyMySQL 0.10.0 and
fixing the up the CI to a working state.

1. Blacklist etcd3gw 0.2.6

etcd3gw 0.2.6 was blacklisted in openstack/requirements [0], because
that version has a bug [1].

tooz does not use openstack/requirements' upper constraints, so the
same blacklisting needs to be introduced here in setup.cfg.

[0] Icb6873d8c5d3a3624c0ac3d76fc9125c5d8134b2
[1] https://github.com/dims/etcd3-gateway/issues/41

Change-Id: I22b955419014dd34c63e406c488e0636ffe9013b
Closes-Bug: #1891314
(cherry picked from commit 3f0759091c)
(cherry picked from commit c8d77ee485)

2. Fix breakage with PyMySQL 0.10.0

In this version a Connection is no longer a context manager. Fix
it by simply getting a Cursor out of it (locks don't seem to
interact with transactions, at least according to MariaDB docs).

Change-Id: I5ea06ebd2b976465ff82f10a74e140f30e9e803f
(cherry picked from commit d59b283440)

releasenotes/notes/mysql-0.10.0-7660f75a1c57a920.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    Fixes AttributeError in the ``mysql`` driver with PyMySQL 0.10.0.

setup.cfg

@@ -51,7 +51,7 @@ etcd3 =
   etcd3>=0.12.0 # Apache-2.0
   grpcio>=1.18.0
 etcd3gw =
-  etcd3gw>=0.1.0 # Apache-2.0
+  etcd3gw!=0.2.6,>=0.1.0 # Apache-2.0
 zake =
   zake>=0.1.6 # Apache-2.0
 redis =

tooz/drivers/mysql.py

@@ -61,12 +61,12 @@ class MySQLLock(locking.Lock):
             try:
                 if not self._conn.open:
                     self._conn.connect()
-                with self._conn as cur:
-                    cur.execute("SELECT GET_LOCK(%s, 0);", self.name)
-                    # Can return NULL on error
-                    if cur.fetchone()[0] is 1:
-                        self.acquired = True
-                        return True
+                cur = self._conn.cursor()
+                cur.execute("SELECT GET_LOCK(%s, 0);", self.name)
+                # Can return NULL on error
+                if cur.fetchone()[0] == 1:
+                    self.acquired = True
+                    return True
             except pymysql.MySQLError as e:
                 utils.raise_with_cause(
                     tooz.ToozError,
@@ -90,9 +90,9 @@ class MySQLLock(locking.Lock):
         if not self.acquired:
             return False
         try:
-            with self._conn as cur:
-                cur.execute("SELECT RELEASE_LOCK(%s);", self.name)
-                cur.fetchone()
+            cur = self._conn.cursor()
+            cur.execute("SELECT RELEASE_LOCK(%s);", self.name)
+            cur.fetchone()
             self.acquired = False
             self._conn.close()
             return True