Read Markdown file as utf8 instead of the default encoding used by OS

### Background
  1. Obsidian stores markdown notes as `utf8`[1]
  2. By default, the python `open` command uses the OS locale encoding[2]

### Issue
  Based on above background, if the OS locale encoding isn't `utf8` it causes the `UnicodeDecodeError: <locale_encoding> codec can't decode byte` error

### Fix
  - Read markdown files as `utf8`
    The Obsidian plugin is the main use-case for markdown files in khoj currently and that stores md files as `utf8`.
    Do not assume utf8 for other content types like org-mode, beancount for now.
  - Fail if error in reading file as utf8, instead of ignoring errors.
    Would rather have user realize that their files are not going to get indexed correctly.

[1]: https://forum.obsidian.md/t/better-handle-md-files-not-stored-in-utf8-format/13524/3
[2]: https://docs.python.org/3/library/functions.html#open

.github/workflows/test.yml

@@ -24,13 +24,20 @@ jobs:
   test:
     name: Run Tests
     runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        python_version:
+          - 3.8
+          - 3.9
+          - 3.10
      steps:
       - uses: actions/checkout@v3
 
-      - name: Set up Python 3.10
+      - name: Set up Python
         uses: actions/setup-python@v4
         with:
-          python-version: '3.10'
+          python-version: ${{ matrix.python_version }}
 
       - name: Install Dependencies
         run: |

src/processor/markdown/markdown_to_jsonl.py

@@ -97,7 +97,7 @@ class MarkdownToJsonl(TextToJsonl):
         entries = []
         entry_to_file_map = []
         for markdown_file in markdown_files:
-            with open(markdown_file) as f:
+            with open(markdown_file, 'r', encoding='utf8') as f:
                 markdown_content = f.read()
                 markdown_entries_per_file = []
                 for entry in re.split(markdown_heading_regex, markdown_content, flags=re.MULTILINE):