Add types to lex.RE #1341

Job	Run time
Documentation	28s
Pylint	36s
Tests for downstream project loopy	4m 5s
Mypy	28s
Tests for downstream project pytato	5m 37s
Pytest on Py3.8 ubuntu-latest	52s
Pytest without Numpy	59s
Pytest on Py3.8 macos-latest	1m 15s
Ruff	4s
Validate CITATION.cff	9s
Pytest on Py3.9 ubuntu-latest	50s
Pytest on Py3.9 macos-latest	54s
Pytest on Py3.x ubuntu-latest	56s
Pytest on Py3.x macos-latest	1m 31s
Pytest on Pypypy3.9 ubuntu-latest	2m 49s
Pytest on Pypypy3.9 macos-latest	7m 38s
	29m 11s

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