ci: migrate from Travis CI to GitHub Actions

- Add ci.yml: lint (ruff + mypy) and test matrix (3.10-3.13, ubuntu/windows/macos)
- Add publish.yml: build + publish to PyPI via trusted publishing (OIDC)
- Remove .travis.yml and old python-publish.yml (username/password auth)
- Replace Travis CI and Coveralls badges with GitHub Actions badge
- Remove coveralls from dev dependencies

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: dsfulf

.github/workflows/ci.yml

@@ -10,8 +10,8 @@ jobs:
   lint:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v4
-    - uses: actions/setup-python@v5
+    - uses: actions/checkout@v5
+    - uses: actions/setup-python@v6
       with:
         python-version: '3.12'
     - name: Install dependencies
@@ -29,8 +29,8 @@ jobs:
         os: [ubuntu-latest, windows-latest, macos-latest]
         python-version: ['3.10', '3.11', '3.12', '3.13']
     steps:
-    - uses: actions/checkout@v4
-    - uses: actions/setup-python@v5
+    - uses: actions/checkout@v5
+    - uses: actions/setup-python@v6
       with:
         python-version: ${{ matrix.python-version }}
     - name: Install dependencies

.github/workflows/publish.yml

@@ -11,8 +11,8 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v4
-    - uses: actions/setup-python@v5
+    - uses: actions/checkout@v5
+    - uses: actions/setup-python@v6
       with:
         python-version: '3.12'
     - name: Install build tools
@@ -21,7 +21,7 @@ jobs:
       run: |
         rm -rf dist/
         python -m build
-    - uses: actions/upload-artifact@v4
+    - uses: actions/upload-artifact@v5
       with:
         name: dist
         path: dist/
@@ -31,7 +31,7 @@ jobs:
     runs-on: ubuntu-latest
     environment: pypi
     steps:
-    - uses: actions/download-artifact@v4
+    - uses: actions/download-artifact@v5
       with:
         name: dist
         path: dist/