Manage requirements with pip-compile

Add a new `requirements.in` file limited to the top-level requirements
the site and build script depend on, and compile that to a fully pinned
`requriements.txt`. [`pip-compile`] has a number of benefits; the main
one this change intends to incorporate is the removal of unused
dependencies from the requirements file.

I plan to follow this change with some additional requirements
automation.

[`pip-compile`]: https://github.com/jazzband/pip-tools#example-usage-for-pip-compile

Author: jonafato

requirements.in

@@ -0,0 +1,3 @@
+httpx
+jinja2
+render_engine

requirements.txt

@@ -1,34 +1,63 @@
-aiohttp==3.8.1
-aiosignal==1.2.0
+#
+# This file is autogenerated by pip-compile with python 3.10
+# To update, run:
+#
+#    pip-compile
+#
 anyio==3.6.1
-async-timeout==4.0.2
-attrs==22.1.0
+    # via httpcore
 certifi==2022.6.15
-charset-normalizer==2.1.1
+    # via
+    #   httpcore
+    #   httpx
 commonmark==0.9.1
-frozenlist==1.3.1
-github==1.2.7
+    # via rich
 h11==0.12.0
+    # via httpcore
 httpcore==0.15.0
+    # via httpx
 httpx==0.23.0
+    # via -r requirements.in
 idna==3.3
-Jinja2==3.1.2
+    # via
+    #   anyio
+    #   rfc3986
+jinja2==3.1.2
+    # via
+    #   -r requirements.in
+    #   render-engine
 markdown2==2.4.3
-MarkupSafe==2.1.1
+    # via render-engine
+markupsafe==2.1.1
+    # via jinja2
 more-itertools==8.14.0
-multidict==6.0.2
+    # via render-engine
 pendulum==2.1.2
-Pygments==2.13.0
+    # via render-engine
+pygments==2.13.0
+    # via rich
 python-dateutil==2.8.2
+    # via pendulum
 python-frontmatter==1.0.0
+    # via render-engine
 python-slugify==6.1.2
+    # via render-engine
 pytzdata==2020.1
-PyYAML==6.0
+    # via pendulum
+pyyaml==6.0
+    # via python-frontmatter
 render-engine==2022.7.2
-rfc3986==1.5.0
+    # via -r requirements.in
+rfc3986[idna2008]==1.5.0
+    # via httpx
 rich==12.5.1
+    # via render-engine
 six==1.16.0
+    # via python-dateutil
 sniffio==1.2.0
+    # via
+    #   anyio
+    #   httpcore
+    #   httpx
 text-unidecode==1.3
-typing_extensions==4.3.0
-yarl==1.8.1
+    # via python-slugify