Specify Python 3.9 in Binder Configuration (#756)

* add runtime.txt file to binder folder with Python 3.9

Signed-off-by: Chris Trevino <darthtrevino@gmail.com>

* use python 3.9 in environment.yml

Signed-off-by: Chris Trevino <darthtrevino@gmail.com>

* remove runtime.txt

Signed-off-by: Chris Trevino <darthtrevino@gmail.com>

Signed-off-by: Chris Trevino <darthtrevino@gmail.com>

binder/environment.yml

@@ -2,7 +2,7 @@ channels:
   - defaults
   - r
 dependencies:
-  - python=3
+  - python=3.9
   - numpy
   - scipy
   - pandas