provide markdown extension to render graphviz

this is server side, and a more standard format, and thus I like it more
than mermaid, which I've been using at work. but, I really wanted a
server-side option (see my manifesto) for drawing relationship graphs,
for D&D stuff of all things.

this adds an optional 'graphviz' feature to package installation which
consequently depends on pydot

requirements/requirements-dev.in

@@ -1,6 +1,7 @@
 -r requirements.in
 
 # testing runner, test reporting, packages used during testing (e.g. requests-mock), etc.
+pydot
 pytest
 pytest-cov
 

requirements/requirements-dev.txt

@@ -6,7 +6,7 @@
 #
 appdirs==1.4.4
     # via virtualenv
-attrs==20.3.0
+attrs==21.2.0
     # via pytest
 bandit==1.6.2
     # via -r requirements/requirements-dev.in
@@ -18,7 +18,7 @@ click==7.1.2
     #   pip-tools
 coverage==5.5
     # via pytest-cov
-distlib==0.3.1
+distlib==0.3.2
     # via virtualenv
 dlint==0.11.0
     # via -r requirements/requirements-dev.in
@@ -42,7 +42,7 @@ flake8-logging-format==0.6.0
     # via -r requirements/requirements-dev.in
 flake8-mutable==1.2.0
     # via -r requirements/requirements-dev.in
-flake8==3.9.1
+flake8==3.9.2
     # via
     #   -r requirements/requirements-dev.in
     #   dlint
@@ -55,11 +55,11 @@ flask==1.1.2
     # via -r requirements/requirements.in
 gitdb==4.0.7
     # via gitpython
-gitpython==3.1.14
+gitpython==3.1.18
     # via bandit
 iniconfig==1.1.1
     # via pytest
-isort==5.8.0
+isort==5.9.1
     # via flake8-isort
 itsdangerous==1.1.0
     # via flask
@@ -80,11 +80,11 @@ packaging==20.9
     #   bleach
     #   pytest
     #   tox
-pbr==5.5.1
+pbr==5.6.0
     # via stevedore
 pep517==0.10.0
     # via pip-tools
-pip-tools==6.1.0
+pip-tools==6.2.0
     # via -r requirements/requirements-dev.in
 pluggy==0.13.1
     # via
@@ -96,15 +96,19 @@ py==1.10.0
     #   tox
 pycodestyle==2.7.0
     # via flake8
-pydocstyle==6.0.0
+pydocstyle==6.1.1
     # via flake8-docstrings
+pydot==1.4.2
+    # via -r requirements/requirements-dev.in
 pyflakes==2.3.1
     # via flake8
 pyparsing==2.4.7
-    # via packaging
-pytest-cov==2.11.1
+    # via
+    #   packaging
+    #   pydot
+pytest-cov==2.12.1
     # via -r requirements/requirements-dev.in
-pytest==6.2.3
+pytest==6.2.4
     # via
     #   -r requirements/requirements-dev.in
     #   pytest-cov
@@ -130,10 +134,11 @@ toml==0.10.2
     # via
     #   pep517
     #   pytest
+    #   pytest-cov
     #   tox
 tox-wheel==0.6.0
     # via -r requirements/requirements-dev.in
-tox==3.23.0
+tox==3.23.1
     # via
     #   -r requirements/requirements-dev.in
     #   tox-wheel
@@ -141,14 +146,16 @@ tzlocal==2.1
     # via -r requirements/requirements.in
 versioneer==0.19
     # via -r requirements/requirements-dev.in
-virtualenv==20.4.3
+virtualenv==20.4.7
     # via tox
 webencodings==0.5.1
     # via bleach
 werkzeug==1.0.1
     # via flask
 wheel==0.36.2
-    # via tox-wheel
+    # via
+    #   pip-tools
+    #   tox-wheel
 
 # The following packages are considered to be unsafe in a requirements file:
 # pip