[Security] Bump markdown2 from 2.3.7 to 2.3.9

Bumps [markdown2](https://github.com/trentm/python-markdown2) from 2.3.7 to 2.3.9. **This update includes a security fix.**
- [Release notes](https://github.com/trentm/python-markdown2/releases)
- [Changelog](https://github.com/trentm/python-markdown2/blob/master/CHANGES.md)
- [Commits](trentm/python-markdown2@2.3.7...2.3.9)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

requirements.txt

@@ -6,19 +6,19 @@
 #
 certifi==2018.11.29       # via requests
 chardet==3.0.4            # via requests
-django-amber==0.8.0
+django-amber==0.8.0       # via -r requirements.in
 django-appconf==1.0.2     # via django-compressor
-django-dotenv==1.4.2
-django-markdown-deux==1.0.5
-django==1.10.4
-django_compressor==2.2
+django-compressor==2.2    # via -r requirements.in
+django-dotenv==1.4.2      # via -r requirements.in
+django-markdown-deux==1.0.5  # via -r requirements.in
+django==1.10.4            # via -r requirements.in, django-amber
 http-crawler==0.1.2       # via django-amber
 idna==2.8                 # via requests
 lxml==3.7.1               # via http-crawler
-markdown2==2.3.7          # via django-markdown-deux
+markdown2==2.3.9          # via django-markdown-deux
 pyyaml==3.12              # via django-amber
 rcssmin==1.0.6            # via django-compressor
-requests==2.21.0
+requests==2.21.0          # via -r requirements.in, django-amber, http-crawler
 rjsmin==1.0.12            # via django-compressor
 tinycss2==0.5             # via http-crawler
 urllib3==1.24.1           # via requests