new SyntaxWarning with python 3.8: invalid escape sequence \e

These warnings are new with python 3.8 (see the "Changed in version 3.8" note at the end of https://docs.python.org/3.8/reference/lexical_analysis.html#string-and-bytes-literals ):

=============================== warnings summary ===============================
tests/test_write.py:8
  /builddir/build/BUILD/tomlkit-0.5.3/tests/test_write.py:8: SyntaxWarning: invalid escape sequence \e
    d = {"foo": "\e\u25E6\r"}

tests/test_write.py:14
  /builddir/build/BUILD/tomlkit-0.5.3/tests/test_write.py:14: SyntaxWarning: invalid escape sequence \e
    assert loads(dumps(d))["foo"] == "\e\u25E6\r"

-- Docs: https://docs.pytest.org/en/latest/warnings.html
=============== 2 failed, 225 passed, 2 warnings in 2.28 seconds ===============

Start with the string literals reported in tests/test_write.py at lines 8 and 14, then run the relevant test file under Python 3.8 or newer. Done means the tests still pass and no invalid escape sequence SyntaxWarning is emitted for these cases.