CI build break due to unpinned lxml version

[ghidalgo3]

We don't pin the version of lxml that we use, and over the weekend that package was upgraded with a breaking change.

Describe the bug
Trying to start either the tiler or stac server produces this import error:

Traceback (most recent call last):
tiler_1        |   File "/opt/conda/envs/myenv/lib/python3.9/multiprocessing/process.py", line 315, in _bootstrap
tiler_1        |     self.run()
tiler_1        |   File "/opt/conda/envs/myenv/lib/python3.9/multiprocessing/process.py", line 108, in run
tiler_1        |     self._target(*self._args, **self._kwargs)
tiler_1        |   File "/opt/conda/envs/myenv/lib/python3.9/site-packages/uvicorn/subprocess.py", line 76, in subprocess_started
tiler_1        |     target(sockets=sockets)
tiler_1        |   File "/opt/conda/envs/myenv/lib/python3.9/site-packages/uvicorn/server.py", line 60, in run
tiler_1        |     return asyncio.run(self.serve(sockets=sockets))
tiler_1        |   File "/opt/conda/envs/myenv/lib/python3.9/asyncio/runners.py", line 44, in run
tiler_1        |     return loop.run_until_complete(main)
tiler_1        |   File "uvloop/loop.pyx", line 1517, in uvloop.loop.Loop.run_until_complete
tiler_1        |   File "/opt/conda/envs/myenv/lib/python3.9/site-packages/uvicorn/server.py", line 67, in serve
tiler_1        |     config.load()
tiler_1        |   File "/opt/conda/envs/myenv/lib/python3.9/site-packages/uvicorn/config.py", line 458, in load
tiler_1        |     self.loaded_app = import_from_string(self.app)
tiler_1        |   File "/opt/conda/envs/myenv/lib/python3.9/site-packages/uvicorn/importer.py", line 24, in import_from_string
tiler_1        |     raise exc from None
tiler_1        |   File "/opt/conda/envs/myenv/lib/python3.9/site-packages/uvicorn/importer.py", line 21, in import_from_string
tiler_1        |     module = importlib.import_module(module_str)
tiler_1        |   File "/opt/conda/envs/myenv/lib/python3.9/importlib/__init__.py", line 127, in import_module
tiler_1        |     return _bootstrap._gcd_import(name[level:], package, level)
tiler_1        |   File "<frozen importlib._bootstrap>", line 1030, in _gcd_import
tiler_1        |   File "<frozen importlib._bootstrap>", line 1007, in _find_and_load
tiler_1        |   File "<frozen importlib._bootstrap>", line 986, in _find_and_load_unlocked
tiler_1        |   File "<frozen importlib._bootstrap>", line 680, in _load_unlocked
tiler_1        |   File "<frozen importlib._bootstrap_external>", line 850, in exec_module
tiler_1        |   File "<frozen importlib._bootstrap>", line 228, in _call_with_frames_removed
tiler_1        |   File "/opt/src/pctiler/pctiler/main.py", line 29, in <module>
tiler_1        |     from pctiler.endpoints import (
tiler_1        |   File "/opt/src/pctiler/pctiler/endpoints/item.py", line 5, in <module>
tiler_1        |     from html_sanitizer.sanitizer import Sanitizer
tiler_1        |   File "/opt/conda/envs/myenv/lib/python3.9/site-packages/html_sanitizer/sanitizer.py", line 8, in <module>
tiler_1        |     import lxml.html.clean
tiler_1        |   File "/opt/conda/envs/myenv/lib/python3.9/site-packages/lxml/html/clean.py", line 18, in <module>
tiler_1        |     raise ImportError(
tiler_1        | ImportError: lxml.html.clean module is now a separate project lxml_html_clean.
tiler_1        | Install lxml[html_clean] or lxml_html_clean directly.

To reproduce
Steps to reproduce the behavior:

1. Start the development servers from main after 3/30/2024.

Expected behavior
Servers start.

Screenshots and shell session dumps
No need.

Additional context
No need.