Fix sdist builds

[ocefpaf]

Closes #339

[ocefpaf]

It is there now and should be uploaded in the next tag:

Run ls -l /home/runner/work/cftime/cftime/dist
total 5228
-rw-r--r-- 1 runner docker  226532 Jun 10 1[4](https://github.com/Unidata/cftime/actions/runs/9449706837/job/26027413304?pr=340#step:3:5):13 cftime-1.6.4-cp312-cp312-macosx_10_9_x86_64.whl
-rw-r--r-- 1 runner docker  209120 Jun 10 14:13 cftime-1.6.4-cp312-cp312-macosx_11_0_arm64.whl
-rw-r--r-- 1 runner docker 1321664 Jun 10 14:13 cftime-1.6.4-cp312-cp312-manylinux_2_17_aarch64.manylinux2014_aarch64.whl
-rw-r--r-- 1 runner docker 1367700 Jun 10 14:13 cftime-1.6.4-cp312-cp312-manylinux_2_17_x86_64.manylinux2014_x86_64.whl
-rw-r--r-- 1 runner docker  178[5](https://github.com/Unidata/cftime/actions/runs/9449706837/job/26027413304?pr=340#step:3:6)76 Jun 10 14:13 cftime-1.6.4-cp312-cp312-win_amd64.whl
-rw-r--r-- 1 runner docker  229558 Jun 10 14:13 cftime-1.[6](https://github.com/Unidata/cftime/actions/runs/9449706837/job/26027413304?pr=340#step:3:7).4-cp38-cp38-macosx_10_9_x86_64.whl
-rw-r--r-- 1 runner docker  211180 Jun 10 14:13 cftime-1.6.4-cp38-cp38-macosx_11_0_arm64.whl
-rw-r--r-- 1 runner docker 1342968 Jun 10 14:13 cftime-1.6.4-cp38-cp38-manylinux_2_1[7](https://github.com/Unidata/cftime/actions/runs/9449706837/job/26027413304?pr=340#step:3:8)_x86_64.manylinux2014_x86_64.whl
-rw-r--r-- 1 runner docker  190013 Jun 10 14:13 cftime-1.6.4-cp38-cp38-win_amd64.whl
-rw-r--r-- 1 runner docker   54601 Jun 10 14:13 cftime-1.6.4.tar.gz

See https://github.com/Unidata/cftime/actions/runs/9449706837/job/26027413304?pr=340