requirements.txt: Install correct umap-learn library

There is a name clash among several umap projects and the wrong umap has
been introduced into requirements.txt.

Ref: https://github.com/koorukuroo/umap/issues/1

requirements.txt

@@ -21,5 +21,5 @@ nose==1.3.7
 cardboardlint==1.3.0
 pylint==2.5.3
 gdown
-umap
+umap-learn
 cython