drop gitpython in favor of scmrepo (#378)

related: iterative/dvc#9100

Should fix #369

- Drops support for Python < 3.8
- Replaces gitpython usage with scmrepo
- Migrates tests to use pytest-test-utils

Public facing `gto.api` interface has not changed in this PR, but
internal GTO API has changed. Ideally these changes should probably be
released as a major version bump.

This PR will require changes in studio - gitpython instances can no
longer be passed into the GTO calls (cc @amritghimire)

.github/workflows/check-test-release.yml

@@ -21,7 +21,7 @@ jobs:
         fetch-depth: 0
     - uses: actions/setup-python@v4
       with:
-        python-version: '3.x'
+        python-version: '3.10'
     - name: set PYSHA
       run: echo "PYSHA=$(python -VV | sha256sum | cut -d' ' -f1)" >> $GITHUB_ENV
     - uses: actions/cache@v3
@@ -37,7 +37,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        python: ["3.7", "3.8", "3.9", "3.10"]
+        python: ["3.8", "3.9", "3.10"]
       fail-fast: false
     timeout-minutes: 10
     steps:
@@ -54,7 +54,6 @@ jobs:
         pip install --upgrade pip setuptools wheel
         pip install pre-commit .[tests]
     - run: pre-commit run pylint -a -v --show-diff-on-failure
-      if: matrix.python != '3.7'
     - name: Run tests
       run: |
         git config --global user.email "[email protected]"
@@ -80,7 +79,7 @@ jobs:
         fetch-depth: 0
     - uses: actions/setup-python@v4
       with:
-        python-version: '3.x'
+        python-version: '3.10'
     - id: dist
       uses: casperdcl/deploy-pypi@v2
       with: