gnu: mercurial: Fix test failures with Python 3.8.2.

* gnu/packages/version-control.scm (mercurial-patch, %mercurial-patches): New
variables.
(mercurial)[source](patches): Add %MERCURIAL-PATCHES.

gnu/packages/version-control.scm

@@ -1411,6 +1411,28 @@ also walk each side of a merge and test those changes individually.")
 control to Git repositories.")
     (license license:gpl2)))
 
+(define (mercurial-patch name revision hash)
+  (origin
+    (method url-fetch)
+    (uri (string-append "https://www.mercurial-scm.org/repo/hg/raw-rev/" revision))
+    (file-name (string-append "mercurial-" name ".patch"))
+    (sha256 (base32 hash))))
+
+(define %mercurial-patches
+  (list
+   ;; These three patches fixes compatibility with the updated gzip module
+   ;; in Python 3.8.2: <https://bz.mercurial-scm.org/show_bug.cgi?id=6284>.
+   (mercurial-patch "python-mtime" "6c36a521572edf3a79ee567b118469b3192037cc"
+                    "0bmm7y40r8s081ws2sjvn1v8kvyfan4a97jl0fhdh7yc2pzxlzqq")
+   (mercurial-patch "indent-gzip" "a23b859ad17dd0a5b9bb37846b69b5e30f99c44c"
+                    "1spscv9dgqv38m7h1liki93ax6w97gxayg17fr7wr6acjdfccpr9")
+   (mercurial-patch "python-gzip" "b7ca03dff14c63d64ad7bfa36a2d0a36a6b62253"
+                    "0p88ffhx0kk21ssrsb156ffhpcb7g8mkwwkmq49qpmbm5ag2paf0")
+   ;; This fixes an incompatibility with os.isfile in Python 3.8:
+   ;; <https://bz.mercurial-scm.org/show_bug.cgi?id=6287>.
+   (mercurial-patch "os-isfile" "6a8738dc4a019da4c9df5c26961aa09d45ce1c68"
+                    "0lr069m12kzrkmr1pmhaxg5lxmdwxabsza61qp1i1q70g7sy8lvy")))
+
 (define-public mercurial
   (package
     (name "mercurial")
@@ -1419,6 +1441,7 @@ control to Git repositories.")
              (method url-fetch)
              (uri (string-append "https://www.mercurial-scm.org/"
                                  "release/mercurial-" version ".tar.gz"))
+             (patches %mercurial-patches)
              (sha256
               (base32
                "1nbjpzjrzgql4hrvslpxwbcgn885ikq6ba1yb4w6p78rw9nzkhgp"))))