Add pull request template

.github/pull_request_template.md

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+<!--
+Link to relevant issues or previous PRs, one per line. Use "fixes" to
+automatically close an issue.
+-->
+
+- fixes #<issue number>
+
+<!--
+Ensure each step in CONTRIBUTING.rst is complete by adding an "x" to
+each box below.
+
+If only docs were changed, these aren't relevant and can be removed.
+-->
+
+Checklist:
+
+- [ ] Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
+- [ ] Add or update relevant docs, in the docs folder and in code.
+- [ ] Ensure PR doesn't contain untouched code reformatting: spaces, etc.
+- [ ] Run `flake8` and fix issues.
+- [ ] Run `pytest` no tests failed. See https://clickhouse-driver.readthedocs.io/en/latest/development.html.