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
```