update requirements.txt

Author: pdebuyl

environment.yml

@@ -16,6 +16,7 @@ dependencies:
 - sphinx>=1.7,<1.8
 - coverage=4.4.2
 - pillow
+- imageio>=2.8
 - pip
 - pip:
   - sphinx-gallery==0.2.0

requirements.txt

@@ -2,7 +2,7 @@
 # libraries before the special comment marked `# For conda, pip installable:`
 numpy>=1.13,<1.17
 scipy==0.19.1
-scikit-learn==0.19
+scikit-learn==0.23
 matplotlib>=2.0,<3.0
 scikit-image>=0.14,<0.15
 sympy>=1.2