Make ipywidgets and scipy properly optional (#100)

Typo on `ipywidgets` was causing the whole jupyter stack to get installed for the base stackstac installation!

Scipy is only needed for `reproject_array`, which isn't in the core API IMO. And xarray already raises a decent error when it's missing.

Author: gjoseph92

poetry.lock

@@ -38,7 +38,7 @@ pystac-client = {version = "^0.3", optional = true}
 python = "^3.8"
 rasterio = "^1.2.3"
 sat-search = {version = "^0.3.0", optional = true}
-scipy = "^1.6.1"
+scipy = {version = "^1.6.1", optional = true}
 sphinx-autodoc-typehints = {version = "^1.11.1", optional = true}
 xarray = ">=0.18,<1"
 
@@ -66,7 +66,7 @@ binder = [
   "distributed",
   "geogif",
   "ipyleaflet",
-  "ipywidets",
+  "ipywidgets",
   "jupyterlab-geojson",
   "jupyter-server-proxy",
   "jupyterlab-system-monitor",
@@ -95,6 +95,7 @@ viz = [
   "matplotlib",
   "mercantile",
   "Pillow",
+  "scipy",
 ]
 
 [build-system]