chore(ci): bump minimum python to 3.8

Python 3.7 is no longer maintained and various deps (e.g., numpy)
contain CVEs at the last version supported on 3.7. Bump the minimum to
3.8 to avoid this CVE being detected in the build.

.travis.yml

@@ -1,4 +1,5 @@
 language: python
+dist: focal
 git:
     depth: false
 services:
@@ -17,12 +18,10 @@ deploy:
           repo: IBM/detect-secrets
 matrix:
     include:
-        - env: TOXENV=py37
-          python: 3.7.13
-          dist: xenial # Required for Python >= 3.7 (travis-ci/travis-ci#9069), the GitHub Travis build will use Python 3.7.1 by default if you provide 3.7 without a patch version and the build will fail with AttributeError: 'str' object has no attribute 'name'
         - env: TOXENV=py38
-          python: 3.8
-          dist: xenial # Required for Python >= 3.7 (travis-ci/travis-ci#9069)
+          python: 3.8.18
+        - env: TOXENV=py39
+          python: 3.9.18
 before_install:
     - echo -e "machine github.com\n  login $GH_ACCESS_TOKEN" >> ~/.netrc # Login to GitHub
     - echo -e "machine github.ibm.com\n  login $GHE_ACCESS_TOKEN" >> ~/.netrc # Login to GitHub Enterprise

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 project = detect_secrets
 # These should match the travis env list
-envlist = py{36,37,38,39}
+envlist = py{38,39}
 skip_missing_interpreters = true
 # disabling this option until latest venv-update version is compatible with pip v21
 # tox_pip_extensions_ext_venv_update = true
@@ -18,14 +18,6 @@ commands =
     coverage report --show-missing --include=detect_secrets/* --fail-under 97
     pre-commit run --all-files --show-diff-on-failure
 
-[testenv:py37]
-commands =
-    coverage erase
-    coverage run -m pytest tests
-    coverage report --show-missing --include=tests/* --fail-under 100
-    coverage report --show-missing --include=detect_secrets/* --fail-under 97
-    # Skip pre-commit hook for py37 since flake8 requires Python >= 3.8.1.
-
 [testenv:venv]
 envdir = venv
 commands =