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

[tautschnig]

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.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6957 (b5ab3a1) into develop (4e141a5) will increase coverage by 0.00%.
The diff coverage is 96.87%.

@@           Coverage Diff            @@
##           develop    #6957   +/-   ##
========================================
  Coverage    77.79%   77.79%           
========================================
  Files         1568     1568           
  Lines       180329   180347   +18     
========================================
+ Hits        140290   140309   +19     
+ Misses       40039    40038    -1     

Impacted Files	Coverage Δ
src/solvers/smt2/smt2_tokenizer.h	96.96% <ø> (ø)
src/solvers/smt2/smt2_dec.cpp	73.27% <90.00%> (+2.10%)	⬆️
src/solvers/smt2/smt2_conv.cpp	67.90% <97.29%> (+0.13%)	⬆️
src/solvers/smt2/smt2_tokenizer.cpp	97.70% <100.00%> (ø)
...rc/solvers/smt2_incremental/smt_to_smt2_string.cpp	94.30% <100.00%> (ø)
unit/solvers/smt2/smt2_conv.cpp	100.00% <100.00%> (ø)
...it/solvers/smt2_incremental/smt_to_smt2_string.cpp	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f83fc54...b5ab3a1. Read the comment docs.