re-run cython to support Python 3.10 + misc patches from the Debian package

[mr-c]

Had to temporarily revert part of commit e94c0d9 to be able to rebuild the khmer/_oxli/*.cpp files

diff --git a/setup.py b/setup.py
index 86291415..eecc8ff0 100755
--- a/setup.py
+++ b/setup.py
@@ -64,9 +64,13 @@ ez_setup.use_setuptools(version="3.4.1")

 CMDCLASS = versioneer.get_cmdclass()

-from setuptools import Extension as CyExtension
-HAS_CYTHON = False
-cy_ext = 'cpp'
+import Cython
+from Cython.Distutils import Extension as CyExtension
+HAS_CYTHON = True
+cy_ext = 'pyx'
+print('*** NOTE: Found Cython, extension files will be '
+      'transpiled if this is an install invocation.',
+      file=sys.stderr)

• Is any new functionality in tested? (This can be checked with
  make clean diff-cover or the CodeCov report that is automatically
  generated following a successful CI build.)
• Was a spellchecker run on the source code and documentation after
  changes were made?
• Have any changes to the command-line interface been explicitly described?
  Only backwards-compatible additions are allowed without a major version
  increment. Changing file formats also requires a major version number
  increment.
• Have any substantial changes been documented in CHANGELOG.md? See
  keepachangelog for more details.
• Do the changes respect streaming I/O? (Are they tested for streaming I/O?)

[mr-c]

I can migrate the Travis CI setup to GitHub Actions, if you want; let me know!