community/rstcheck: fix intermittent test failures

Add a patch to close the multiprocessing pool in line with the Python 3
documentation.

The patch introduced by this commit has been submitted upstream:
https://github.com/myint/rstcheck/pull/67

Testing inside `dabuild sh`, before this change we can't get above run 11:

```
$ cd src/rstcheck-3.3.1/
$ for i in $(seq 99) ; do printf '%3d' $i && ./rstcheck.py --recursive examples/good || break ; done ; echo
  1 (stalled)
$ for i in $(seq 99) ; do printf '%3d' $i && ./rstcheck.py --recursive examples/good || break ; done ; echo
  1  2  3  4  5  6  7  8  9 10 11 (stalled)
$ for i in $(seq 99) ; do printf '%3d' $i && ./rstcheck.py --recursive examples/good || break ; done ; echo
  1  2  3 (stalled)
```

After this change, 99 runs finish three times in a row:

```
$ abuild deps unpack prepare build
$ cd src/rstcheck-3.3.1/
$ for i in $(seq 99) ; do printf '%3d' $i && ./rstcheck.py --recursive examples/good || break ; done ; echo
  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99
$ for i in $(seq 99) ; do printf '%3d' $i && ./rstcheck.py --recursive examples/good || break ; done ; echo
  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99
$ for i in $(seq 99) ; do printf '%3d' $i && ./rstcheck.py --recursive examples/good || break ; done ; echo
  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99
```

1 files changed, 59 insertions, 0 deletions

diff --git a/community/rstcheck/02-Close-the-multiprocessing.pool-after-use.patch b/community/rstcheck/02-Close-the-multiprocessing.pool-after-use.patch
new file mode 100644
index 00000000000..d0680ef23df
--- /dev/null
+++ b/community/rstcheck/02-Close-the-multiprocessing.pool-after-use.patch
@@ -0,0 +1,59 @@
+https://github.com/myint/rstcheck/pull/67
+
+From 5e6e995305f66246e8f98acd15de2c39a75efa5b Mon Sep 17 00:00:00 2001
+From: Keith Maxwell <keith.maxwell@gmail.com>
+Date: Mon, 11 May 2020 20:53:14 +0100
+Subject: [PATCH] Close the multiprocessing.pool after use
+
+So that the tests do not hang intermittently
+
+When preparing to release Alpine Linux 3.12, the tests for this package
+would hang intermittently. This problem was isolated to the tests in
+test.bash that run rstcheck.py over multiple files.
+
+The documentation for multiprocessing.pool explains that:
+
+> Warning multiprocessing.pool objects have internal resources that need
+> to be properly managed (like any other resource) by using the pool as
+> a context manager or by calling close() and terminate() manually.
+> Failure to do this can lead to the process hanging on finalization.
+
+> Note that is not correct to rely on the garbage colletor to destroy
+> the pool as CPython does not assure that the finalizer of the pool
+> will be called (see object.__del__() for more information).
+
+https://docs.python.org/3/library/multiprocessing.html#module-multiprocessing.pool
+
+Before this commit close() was not called on the multiprocessing pool;
+after this commit close() is called.
+
+The change in this commit was tested in an Alpine Linux container:
+
+    podman run -ti --rm -v $PWD:/srv:Z -w /srv alpine:edge sh
+
+By running:
+
+    apk add alpine-sdk python3 py3-docutils py3-setuptools bash
+    python3 setup.py build
+    sed -i '1s|^#!/usr/bin/env python$|#!/usr/bin/python3|' rstcheck.py
+    python3 ./test_rstcheck.py
+    bash ./test.bash
+---
+ rstcheck.py | 1 +
+ 1 file changed, 1 insertion(+)
+
+diff --git a/rstcheck.py b/rstcheck.py
+index a13ec2b..959808a 100755
+--- a/rstcheck.py
++++ b/rstcheck.py
+@@ -981,6 +981,7 @@ def main():
+         except (IOError, UnicodeError) as exception:
+             output_message(exception)
+             status = 1
++        pool.close()
+ 
+         return status
+ 
+-- 
+2.26.2
+