Try looking for EXT_SUFFIX not SO

The 3.10 travisci run complained that "SO" was depricated and
"EXT_SUFFIX" should be used instead.

Maybe it's the varname not distutils deprecation causing the issue?
In any case clear distutils just on principles.
Plus add debugging steps

Author: rouilj

.travis.yml

@@ -70,10 +70,10 @@ before_install:
   #  doesn't work for 3.11 or newer.
   # Change distutils.sysconfig... to just sysconfig to get valid SO.
   - python3 -V
-  - if [[ $TRAVIS_PYTHON_VERSION == "3."* ]]; then sed -i '/PYTHON3_SO=/s/distutils.//g' configure; grep PYTHON3_SO configure; ./configure --prefix=$VIRTUAL_ENV --with-python3 --disable-documentation; fi
-  - if [[ $TRAVIS_PYTHON_VERSION == "nightly" ]]; then sed -i '/PYTHON3_SO=/s/distutils.//g' configure; grep PYTHON3_SO configure; ./configure --prefix=$VIRTUAL_ENV --with-python3 --disable-documentation; fi
-  - if [[ $TRAVIS_PYTHON_VERSION == "pypy3" ]]; then sed -i '/PYTHON3_SO=/s/distutils.//g' configure; ./configure --prefix=$VIRTUAL_ENV --with-python3 --disable-documentation; fi
-  - python3 -c 'import sysconfig; s=sysconfig.get_config_var("SO"); print(s) if s else exit(1)'
+  - if [[ $TRAVIS_PYTHON_VERSION == "3."* ]]; then sed -i -e '/PYTHON3_SO=/s/distutils\.//g' -e '/PYTHON3_SO=/s/"SO"/"EXT_SUFFIX"/g' configure; grep PYTHON3_SO configure; ./configure --prefix=$VIRTUAL_ENV --with-python3 --disable-documentation; fi
+  - if [[ $TRAVIS_PYTHON_VERSION == "nightly" ]]; then sed -i -e '/PYTHON3_SO=/s/distutils\.//g' -e '/PYTHON3_SO=/s/"SO"/"EXT_SUFFIX"/g' configure; grep PYTHON3_SO configure; ./configure --prefix=$VIRTUAL_ENV --with-python3 --disable-documentation; fi
+  - if [[ $TRAVIS_PYTHON_VERSION == "pypy3" ]]; then sed -i -e '/PYTHON3_SO=/s/distutils\.//g' -e '/PYTHON3_SO=/s/"SO"/"EXT_SUFFIX"/g' configure; ./configure --prefix=$VIRTUAL_ENV --with-python3 --disable-documentation; fi
+  - python3 -c 'import sysconfig; s=sysconfig.get_config_var("EXT_SUFFIX"); print(s) if s else exit(1)'
   # - case "$TRAVIS_PYTHON_VERSION" in nightly|3.11-dev) echo skipping xapian build;; *) make && make install; esac
   - make && make install