MAINT: use placeholder email address for Thomas Li in pyproject.toml

This is the same email address that is used in the for the git
commits. This should fix how the Maintainers metadata field is
rendered on the PyPI package page.

pyproject.toml

@@ -23,7 +23,7 @@ maintainers = [
   { name = 'Ralf Gommers ', email = '[email protected]' },
   { name = 'Daniele Nicolodi', email = '[email protected]' },
   { name = 'Henry Schreiner', email = '[email protected]' },
-  { name = 'Thomas Li'},
+  { name = 'Thomas Li', email = '[email protected]' },
 ]
 classifiers = [
   'Development Status :: 5 - Production/Stable',