[3.7] bpo-33488: Satisfy markdownlint for the pull request template. (GH-6786)

./.github/PULL_REQUEST_TEMPLATE.md:8: MD031 Fenced code blocks should be
surrounded by blank lines
./.github/PULL_REQUEST_TEMPLATE.md:10: MD031 Fenced code blocks should
be surrounded by blank lines
./.github/PULL_REQUEST_TEMPLATE.md:19: MD031 Fenced code blocks should
be surrounded by blank lines
./.github/PULL_REQUEST_TEMPLATE.md:21: MD031 Fenced code blocks should
be surrounded by blank lines.
(cherry picked from commit 5cd22cf)

Co-authored-by: Eitan Adler <[email protected]>

Author: benjaminp

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,8 +1,10 @@
 !!! If this is a backport PR (PR made against branches other than `master`),
 please ensure that the PR title is in the following format:
+
 ```
 [X.Y] <title from the original PR> (GH-NNNN)
 ```
+
 Where: [X.Y] is the branch name, e.g. [3.6].
 
 GH-NNNN refers to the PR number from `master`.