ci: twisted and oldattrs tox envs are now incompatible, don't run them together

twisted started to use `attr.s(eq)` argument which was added recently,
so it fails with oldattrs. One of the CI jobs ran twisted and oldattrs
together, so it started to fail.

Move the twisted code to be covered by another job, and remove it from
the job with the oldattrs.

(cherry picked from commit 2cc3227)

Author: bluetech

.github/workflows/main.yml

@@ -62,15 +62,15 @@ jobs:
           - name: "windows-py37"
             python: "3.7"
             os: windows-latest
-            tox_env: "py37-twisted-numpy"
+            tox_env: "py37-numpy"
           - name: "windows-py37-pluggy"
             python: "3.7"
             os: windows-latest
             tox_env: "py37-pluggymaster-xdist"
           - name: "windows-py38"
             python: "3.8"
             os: windows-latest
-            tox_env: "py38"
+            tox_env: "py38-twisted"
             use_coverage: true
 
           - name: "ubuntu-py35"
@@ -84,7 +84,7 @@ jobs:
           - name: "ubuntu-py37"
             python: "3.7"
             os: ubuntu-latest
-            tox_env: "py37-lsof-numpy-oldattrs-pexpect-twisted"
+            tox_env: "py37-lsof-numpy-oldattrs-pexpect"
             use_coverage: true
           - name: "ubuntu-py37-pluggy"
             python: "3.7"