Force installation only from piwheels

Forcing the use of piwheels avoids trying
to install numpy/scipy from source,
which is the Right Answer.

Unfortunately, sometimes packages fail on piwheels, but poetry still
tries to install them.  I've had to fix
pyyaml at 5.4.0 for that reason.

pyproject.toml

@@ -28,21 +28,31 @@ packages = [
     { include = "openflexure_microscope" },
 ]
 
+# Installing numpy/scipy/etc. from source is unlikely to work.
+# The block below forces Poetry to use piwheels.
+# We can add other repositories if we need them.
+# Setting default=true disables fallback to PyPI.
+
+[[tool.poetry.source]]
+name = "piwheels"
+url = "https://www.piwheels.org/simple"
+default = true
+
 [tool.poetry.dependencies]
 python = "^3.7.3"
 Flask = "^1.0"
 numpy = "1.19.2" # Exact version so we can guarantee a wheel
 Pillow = "^7.2.0"
-scipy = "1.5.3" # Exact version so we can guarantee a wheel
+scipy = "1.6.1" # Exact version so we can guarantee a wheel
 "RPi.GPIO" = { version = "^0.7.0", optional = true }
 python-dateutil = "^2.8"
 psutil = "^5.6.7" # Autostorage extension
-opencv-python-headless = "4.4.0.44"
+opencv-python-headless = "4.5.1.48"
 sangaboard = "^0.2"
 expiringdict = "^1.2.1"
 camera-stage-mapping = "0.1.4"
 picamerax = ">=20.9.1"
-pyyaml = "^5.3.1"
+pyyaml = "5.4.0" # pyyaml 5.4.1 is broken in piwheels
 pytest-cov = "^2.10.1"
 piexif = "^1.1.3"
 labthings = "^1.2.2"
@@ -64,7 +74,7 @@ pytest = "^6.1.2"
 mypy = "^0.790"
 poethepoet = "^0.9.0"
 freezegun = "^1.0.0"
-lxml = "^4.6.2"
+lxml = "^4.6.2" # Fix version so piwheels works
 
 [tool.black]
 exclude = '(\.eggs|\.git|\.venv|\venv|node_modules/)'