Use Python 3.8 for CI builds (#988)

### Public-Facing Changes

Use Python 3.8 for CI builds 

### Description
Ensures that Python3.8 is used for CI. This was not the case so far as
`pipenv` was "resetting" the python version:

```
make lint
  shell: /usr/bin/bash -e {0}
  env:
    pythonLocation: /opt/hostedtoolcache/Python/3.7.17/x64
    PKG_CONFIG_PATH: /opt/hostedtoolcache/Python/3.7.17/x64/lib/pkgconfig
    Python_ROOT_DIR: /opt/hostedtoolcache/Python/3.7.17/x64
    Python2_ROOT_DIR: /opt/hostedtoolcache/Python/3.7.17/x64
    Python3_ROOT_DIR: /opt/hostedtoolcache/Python/3.7.17/x64
    LD_LIBRARY_PATH: /opt/hostedtoolcache/Python/3.7.17/x64/lib
SETUPTOOLS_ENABLE_FEATURES="legacy-editable" pipenv install --dev --deploy
Creating a virtualenv for this project...
Pipfile: /home/runner/work/mcap/mcap/python/Pipfile
Using /usr/bin/python3 (3.10.12) to create virtualenv...
created virtual environment CPython3.10.12.final.0-64 in 636ms
```

.github/workflows/ci.yml

@@ -248,7 +248,7 @@ jobs:
           lfs: true
       - uses: actions/setup-python@v4
         with:
-          python-version: 3.7
+          python-version: 3.8
       - run: pip install pipenv==2023.2.4
       - run: make lint
       - run: make test

.github/workflows/website.yml

@@ -55,7 +55,7 @@ jobs:
 
       - uses: actions/setup-python@v4
         with:
-          python-version: 3.7
+          python-version: 3.8
 
       - run: pip install pipenv
 

python/Pipfile

@@ -35,4 +35,4 @@ typing_extensions = "*"
 isort = "*"
 
 [requires]
-python_version = "3"
+python_version = "3.8"