Enable Python 3.10's EncodingWarning

This warns on the default encoding being used in ``open`` or ``io.open``,
which can lead to cross-platform bugs (for example on Windows where the
default codepage is cp1252), or with the upcoming change to the default Python
encoding (PEP 686, Make UTF-8 mode default https://peps.python.org/pep-0686/)

Author: AA-Turner

tox.ini

@@ -33,6 +33,10 @@ passenv = USER USERNAME COVERAGE_* PYTEST_ADDOPTS TERM SETUPTOOLS_SCM_PRETEND_VE
 setenv =
     _PYTEST_TOX_DEFAULT_POSARGS={env:_PYTEST_TOX_POSARGS_DOCTESTING:} {env:_PYTEST_TOX_POSARGS_LSOF:} {env:_PYTEST_TOX_POSARGS_XDIST:}
 
+    # Warn where default encoding is used
+    # https://docs.python.org/3/library/io.html#io-encoding-warning
+    PYTHONWARNDEFAULTENCODING=1
+
     # Configuration to run with coverage similar to CI, e.g.
     # "tox -e py37-coverage".
     coverage: _PYTEST_TOX_COVERAGE_RUN=coverage run -m