Repository: PyGithub/PyGithub

Issue:
get_review_comments() method fails due to missing import
Specifically this line:
https://github.com/PyGithub/PyGithub/blob/master/github/PullRequest.py#L552
requires that the `datetime` module be loaded. It isn't.

Adding a 
```python
import datetime
```
at the top of `github/PullRequest.py` fixes this bug.

I checked the test coverage and indeed there doesn't seem to be a test of this method.



Discussion:
👍 

Resolving patch:
diff --git a/github/PullRequest.py b/github/PullRequest.py
--- a/github/PullRequest.py
+++ b/github/PullRequest.py
@@ -42,6 +42,7 @@
 #                                                                              #
 ################################################################################
 
+import datetime
 import urllib
 import github.GithubObject
 import github.PaginatedList


Test patch:
diff --git a/github/tests/PullRequest.py b/github/tests/PullRequest.py
--- a/github/tests/PullRequest.py
+++ b/github/tests/PullRequest.py
@@ -131,6 +131,11 @@ def testGetIssueComment(self):
         comment = self.pull.get_issue_comment(8387331)
         self.assertEqual(comment.body, "Issue comment created by PyGithub")
 
+    def testGetReviewComments(self):
+        epoch = datetime.datetime(1970, 1, 1, 0, 0)
+        comments = self.pull.get_review_comments(since=epoch)
+        self.assertListKeyEqual(comments, lambda c: c.id, [238127783])
+
     def testReviewRequests(self):
         self.pull.create_review_request(reviewers="sfdye", team_reviewers="pygithub-owners")
         review_requests = self.pull.get_review_requests()
diff --git a/github/tests/ReplayData/PullRequest.testGetReviewComments.txt b/github/tests/ReplayData/PullRequest.testGetReviewComments.txt
new file mode 100644
--- /dev/null
+++ b/github/tests/ReplayData/PullRequest.testGetReviewComments.txt
@@ -0,0 +1,10 @@
+https
+GET
+api.github.com
+None
+/repos/jacquev6/PyGithub/pulls/31/comments?since=1970-01-01T00%3A00%3A00Z
+{'Authorization': 'Basic login_and_password_removed', 'User-Agent': 'PyGithub/Python'}
+None
+200
+[('vary', 'Accept, Authorization, Cookie, X-GitHub-OTP, Accept-Encoding'), ('x-oauth-scopes', 'public_repo, repo:status'), ('x-xss-protection', '1; mode=block'), ('x-content-type-options', 'nosniff'), ('x-accepted-oauth-scopes', ''), ('etag', 'W/"a120ecdfea5b9aa9b12d61e596663d10"'), ('cache-control', 'private, max-age=60, s-maxage=60'), ('referrer-policy', 'origin-when-cross-origin, strict-origin-when-cross-origin'), ('status', '200 OK'), ('x-ratelimit-remaining', '4960'), ('x-github-media-type', 'github.v3; format=json'), ('access-control-expose-headers', 'ETag, Link, Location, Retry-After, X-GitHub-OTP, X-RateLimit-Limit, X-RateLimit-Remaining, X-RateLimit-Reset, X-OAuth-Scopes, X-Accepted-OAuth-Scopes, X-Poll-Interval, X-GitHub-Media-Type'), ('transfer-encoding', 'chunked'), ('x-github-request-id', 'B86E:30A2:A2F3D3D:CC9EB57:5C2EE8ED'), ('date', 'Fri, 04 Jan 2019 05:02:44 GMT'), ('access-control-allow-origin', '*'), ('content-security-policy', "default-src 'none'"), ('content-encoding', 'gzip'), ('strict-transport-security', 'max-age=31536000; includeSubdomains; preload'), ('server', 'GitHub.com'), ('x-ratelimit-limit', '5000'), ('x-frame-options', 'deny'), ('content-type', 'application/json; charset=utf-8'), ('x-ratelimit-reset', '1546581173')]
+[{"original_position":4,"diff_hunk":"@@-39,6+39,15@@Getorganizationbyname\\n>>>org.login\\n'PyGithub'\\n\\n+Getresponseaspythondictionary","pull_request_url":"https://api.github.com/repos/jacquev6/PyGithub/pulls/31","url":"https://api.github.com/repos/jacquev6/PyGithub/pulls/comments/238127783","created_at":"2018-12-03T01:52:16Z","author_association":"MEMBER","body":"```suggestion\\r\\nGetrawresponsepythondictionary\\r\\n```","updated_at":"2018-12-03T01:52:16Z","html_url":"https://github.com/jacquev6/PyGithub/pull/975#discussion_r238127783","pull_request_review_id":180593653,"_links":{"self":{"href":"https://api.github.com/repos/jacquev6/PyGithub/pulls/comments/238127783"},"html":{"href":"https://github.com/jacquev6/PyGithub/pull/31#discussion_r238127783"},"pull_request":{"href":"https://api.github.com/repos/jacquev6/PyGithub/pulls/31"}},"node_id":"MDI0OlB1bGxSZXF1ZXN0UmV2aWV3Q29tbWVudDIzODEyNzc4Mw==","commit_id":"be3977efe5fcc7affc3f1c1fc4b6b8add7465010","user":{"gravatar_id":"b68de5ae38616c296fa345d2b9df2225","login":"jacquev6","url":"https://api.github.com/users/jacquev6","id":327146,"avatar_url":"https://secure.gravatar.com/avatar/b68de5ae38616c296fa345d2b9df2225?d=https://a248.e.akamai.net/assets.github.com%2Fimages%2Fgravatars%2Fgravatar-user-420.png"},"position":4,"path":"doc/examples/MainClass.rst","original_commit_id":"be3977efe5fcc7affc3f1c1fc4b6b8add7465010","id":238127783}]