gnu: Add snakemake-4.

* gnu/packages/python.scm (snakemake-4): New variable.
* gnu/packages/bioinformatics.scm (pigx-bsseq, pigx-rnaseq, pigx-scrnaseq,
pigx-chipseq)[inputs]: Use it.

gnu/packages/python.scm

@@ -5206,6 +5206,24 @@ providing a clean and modern domain specific specification language (DSL) in
 Python style, together with a fast and comfortable execution environment.")
     (license license:expat)))
 
+;; This is currently needed for the pigx-* packages.
+(define-public snakemake-4
+  (package (inherit snakemake)
+    (version "4.4.0")
+    (source
+     (origin
+       (method url-fetch)
+       (uri (pypi-uri "snakemake" version))
+       (sha256
+        (base32 "0g0paia4z7w3srnqdmavq3hrb2x7qnpf81jx50njl0p7y4y0j8jv"))))
+    (propagated-inputs
+     `(("python-wrapt" ,python-wrapt)
+       ("python-requests" ,python-requests)
+       ("python-appdirs" ,python-appdirs)
+       ("python-configargparse" ,python-configargparse)
+       ("python-pyyaml" ,python-pyyaml)
+       ("python-ratelimiter" ,python-ratelimiter)))))
+
 (define-public python-pyqrcode
   (package
     (name "python-pyqrcode")