Update version requirements for compiling docs

and update versions in requirements.txt too (for CI)

Author: max-veit

README.rst

@@ -92,11 +92,11 @@ The following packages are required for building some optional features:
 +==================+=============+====================+
 | Documentation    | pandoc      | (latest)           |
 +------------------+-------------+--------------------+
-|                  | sphinx      | 2.1.2              |
+|                  | sphinx      | 2.1.2 or later     |
 +------------------+-------------+--------------------+
-|                  | breathe     | 4.13.1             |
+|                  | breathe     | 4.14.1 or later    |
 +------------------+-------------+--------------------+
-|                  | nbsphinx    | (latest)           |
+|                  | nbsphinx    | 0.8.1 or later     |
 +------------------+-------------+--------------------+
 
 Compiling

requirements.txt

@@ -7,10 +7,10 @@ matplotlib
 #qml
 # tqdm
 # for documentation
-sphinx==2.1.2
-breathe==4.13.1
+sphinx>=2.1.2
+breathe>=4.14.1
 sphinx_rtd_theme
-nbsphinx
+nbsphinx>=0.8.1
 pygments>=2.4.1
 # Developers tools - code formatting
 cpplint