C++17 and Python 3.12

[sarnold]

There's some good news:

• changing cmake to c++17 and extensions ON seems to work fine
  and some not-so-good news:
• SyntaxWarnings in py 3.12

=============================== warnings summary ===============================
tests/test_charliterals.txt::test_charliterals.txt
  <doctest test_charliterals.txt[12]>:1: SyntaxWarning: invalid escape sequence '\9'
tests/test_charliterals.txt::test_charliterals.txt
tests/test_charliterals.txt::test_charliterals.txt
  <unknown>:1: SyntaxWarning: invalid escape sequence '\9'
tests/test_charliterals.txt::test_charliterals.txt
  <doctest test_charliterals.txt[19]>:1: SyntaxWarning: invalid escape sequence '\9'
tests/test_search.txt::test_search.txt
  <doctest test_search.txt[4]>:1: SyntaxWarning: invalid escape sequence '\d'
tests/test_search.txt::test_search.txt
  <doctest test_search.txt[5]>:1: SyntaxWarning: invalid escape sequence '\d'
tests/test_sub.txt::test_sub.txt
  <doctest test_sub.txt[7]>:1: SyntaxWarning: invalid escape sequence '\('
-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
======================== 74 passed, 7 warnings in 1.52s ========================

[sarnold]

@andreasvc fixes in #47