Minor python environment changes (#723)

Substitutes python-memcached (which is installable out of the box on Windows) for pylibmc (which isn't), only on Windows
Adds a git ignore for the common venv subdirectory for a working virtual environment when doing development

Author: davidfraser

.gitignore

@@ -19,3 +19,4 @@ __pycache__/
 api-test.*
 notes.md
 notes.py
+venv/

requirements.txt

@@ -18,11 +18,12 @@ pillow~=9.3.0
 psycopg~=3.1.8
 pydantic~=1.10.2
 pyld~=2.0.3
-pylibmc~=1.6.3
+pylibmc~=1.6.3; sys_platform != 'win32'
 pymemcache~=4.0.0
 pytest-asyncio~=0.20.3
 python-dateutil~=2.8.2
 python-dotenv~=0.21.0
+python-memcached~=1.62; sys_platform == 'win32'
 redis~=4.4.0
 sentry-sdk~=1.11.0
 sphinx~=5.3.0