Update linters and adjust dependency versions

.github/workflows/main.yml

@@ -49,6 +49,10 @@ jobs:
 
     steps:
       - uses: actions/checkout@v2
+      - name: Validate links in Markdown files
+        uses: JustinBeckwith/linkinator-action@v1
+        with:
+          retry: true
       - name: Set up Python
         uses: actions/setup-python@v2
         with:

pyproject.toml

@@ -28,19 +28,19 @@ classifiers = [
 "Issue Tracker" = "https://github.com/pelican-plugins/share-post/issues"
 
 [tool.poetry.dependencies]
-python = "^3.6"
-pelican = "^4.5"
-markdown = {version = "^3.2.2", optional = true}
+python = ">=3.6.2,<4.0"
+pelican = ">=4.5"
+markdown = {version = ">=3.2", optional = true}
 beautifulsoup4 = "^4.9.3"
 
 [tool.poetry.dev-dependencies]
-black = {version = "^19.10b0", allow-prereleases = true}
+black = {version = "^21.4b2", allow-prereleases = true}
 flake8 = "^3.9"
 flake8-black = "^0.2.0"
 invoke = "^1.3"
 isort = "^5.4"
 livereload = "^2.6"
-markdown = "^3.2.2"
+markdown = "^3.2"
 pytest = "^6.0"
 pytest-cov = "^2.8"
 pytest-pythonpath = "^0.7.3"