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

[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 5cd22cf2097a6dd209999cf97858d54564a3be87)

Co-authored-by: Eitan Adler <grimreaper@users.noreply.github.com>