I've converted _sre to multi-phase init in this branch: https://github.com/erlend-aasland/cpython/tree/bpo-1635741/sre

I'm waiting for GH-23101 to be merged (or rejected) before I submit the PR. It's a fairly large PR (1 file changed, 259 insertions(+), 173 deletions(-), ex. clinic code). Let me know if you want me to split it up.

Here's an overview over the commits in the branch (clinic included):
97fc9aad3f (HEAD -> bpo-1635741/sre, origin/bpo-1635741/sre) Convert _sre to multi-phase initialisation
 1 file changed, 55 insertions(+), 15 deletions(-)
f2cc4bdf45 Convert global state to module state
 2 files changed, 212 insertions(+), 345 deletions(-)
0d9b3cb47e Establish global module state and add types to it
 1 file changed, 28 insertions(+), 14 deletions(-)
823767cf9a Convert _sre scanner type to heap type
 1 file changed, 19 insertions(+), 37 deletions(-)
a018a9ce15 Convert _sre match type to heap type
 1 file changed, 31 insertions(+), 43 deletions(-)
7e6e997b59 Convert _sre pattern type to heap type
 1 file changed, 35 insertions(+), 44 deletions(-)
06d23e377c Add convenience macro
 1 file changed, 12 insertions(+)