Lock pelican to 4.2.0 to prevent rendering bugs

Unable to import ansilexer from m.code. This breaks all syntax
highlighting and also causes several articles not to render
at all.

Suggestion at mosra/m.css#181 is to pin
the pelican version for now.

src/requirements.in

@@ -1,4 +1,4 @@
 beautifulsoup4
 invoke
-pelican[markdown]
+pelican[markdown]==4.2.0
 pelican-series-plugin

src/requirements.txt

@@ -16,13 +16,13 @@ invoke==1.5.0
     # via -r requirements.in
 jinja2==2.11.3
     # via pelican
-markdown==3.3.4
+markdown==3.1.1
     # via pelican
 markupsafe==1.1.1
     # via jinja2
 pelican-series-plugin==1.1.1
     # via -r requirements.in
-pelican[markdown]==4.6.0
+pelican[markdown]==4.2.0
     # via
     #   -r requirements.in
     #   pelican-series-plugin
@@ -37,8 +37,12 @@ pytz==2021.1
 six==1.15.0
     # via
     #   feedgenerator
+    #   pelican
     #   python-dateutil
 soupsieve==2.2.1
     # via beautifulsoup4
 unidecode==1.2.0
     # via pelican
+
+# The following packages are considered to be unsafe in a requirements file:
+# setuptools