[Security] Bump lxml from 3.7.1 to 4.6.3

Bumps [lxml](https://github.com/lxml/lxml) from 3.7.1 to 4.6.3. **This update includes security fixes.**
- [Release notes](https://github.com/lxml/lxml/releases)
- [Changelog](https://github.com/lxml/lxml/blob/master/CHANGES.txt)
- [Commits](lxml/lxml@lxml-3.7.1...lxml-4.6.3)

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

Author: dependabot-preview[bot]

requirements.txt

@@ -4,22 +4,46 @@
 #
 #    pip-compile --output-file requirements.txt requirements.in
 #
-certifi==2018.11.29       # via requests
-chardet==3.0.4            # via requests
+certifi==2018.11.29
+    # via requests
+chardet==3.0.4
+    # via requests
 django-amber==0.8.0
-django-appconf==1.0.2     # via django-compressor
+    # via -r requirements.in
+django-appconf==1.0.2
+    # via django-compressor
+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
-django_compressor==2.2
-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
-pyyaml==3.12              # via django-amber
-rcssmin==1.0.6            # via django-compressor
+    # via
+    #   -r requirements.in
+    #   django-amber
+http-crawler==0.1.2
+    # via django-amber
+idna==2.8
+    # via requests
+lxml==4.6.3
+    # via http-crawler
+markdown2==2.3.7
+    # via django-markdown-deux
+pyyaml==3.12
+    # via django-amber
+rcssmin==1.0.6
+    # via django-compressor
 requests==2.21.0
-rjsmin==1.0.12            # via django-compressor
-tinycss2==0.5             # via http-crawler
-urllib3==1.24.1           # via requests
-webencodings==0.5         # via tinycss2
+    # 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
+webencodings==0.5
+    # via tinycss2