Update cpplint to cpplint/cpplint@2cba6ce8dfc for Python 3 support

This updates the script to the latest version (2cba6ce8dfc) of the
much-more-actively developed fork in https://github.com/cpplint/cpplint.
The main reason for this update is to finally have support for Python 3
(even though Python 2 will still work): not all of our CI environments
even have Python 2 anymore.

All local changes were retained. As documented in the test, the only
visible change in behaviour is that operator<< spacing checks now
actually work.

Author: tautschnig

.github/workflows/pull-request-checks.yaml

@@ -254,7 +254,7 @@ jobs:
           DEBIAN_FRONTEND: noninteractive
         run: |
           sudo apt-get update
-          sudo apt-get install --no-install-recommends -yq cmake ninja-build gcc gdb g++ maven flex bison libxml2-utils dpkg-dev ccache doxygen z3 python2
+          sudo apt-get install --no-install-recommends -yq cmake ninja-build gcc gdb g++ maven flex bison libxml2-utils dpkg-dev ccache doxygen z3
       - name: Confirm z3 solver is available and log the version installed
         run: z3 --version
       - name: Download cvc-5 from the releases page and make sure it can be deployed

regression/cpp-linter/operator-spacing2/main.cpp

@@ -24,10 +24,8 @@ static void fun()
 
   int x = 1<<4;
 
-  // Ideally this should produce an error, see operator-spacing3
   status()<<"Adding CPROVER library ("<<eom;
 
-  // Ideally this should produce an error, see operator-spacing3
   int x = 1 << 4;
 
   int y = a<<b;

regression/cpp-linter/operator-spacing2/test.desc

@@ -1,7 +1,9 @@
 CORE
 main.cpp
 
-^# Total errors found: 0$
-^EXIT=0$
+^regression/cpp-linter/operator-spacing2/main\.cpp:27:  Missing spaces around <<  \[whitespace/operators\] \[3\]
+^regression/cpp-linter/operator-spacing2/main\.cpp:31:  Missing spaces around <<  \[whitespace/operators\] \[3\]
+^# Total errors found: 2$
+^EXIT=1$
 ^SIGNAL=0$
 --

regression/cpp-linter/operator-spacing3/test.desc

@@ -1,11 +1,9 @@
-KNOWNBUG
+CORE
 main.cpp
 
-^main\.cpp:27:  Missing spaces around <<  \[whitespace/operators\] \[3\]
-^main\.cpp:27:  Missing spaces around <<  \[whitespace/operators\] \[3\]
-^main\.cpp:29:  Remove spaces around <<  \[whitespace/operators\] \[4\]
-^main\.cpp:29:  Remove spaces around <<  \[whitespace/operators\] \[4\]
-^# Total errors found: 4$
+^regression/cpp-linter/operator-spacing3/main\.cpp:27:  Missing spaces around <<  \[whitespace/operators\] \[3\]
+^regression/cpp-linter/operator-spacing3/main\.cpp:31:  Missing spaces around <<  \[whitespace/operators\] \[3\]
+^# Total errors found: 2$
 ^EXIT=1$
 ^SIGNAL=0$
 --