Update conda env description & pkg versions

This resolves the security alerts identified by dependabot when
the repo was migrated to GitHub:
* jinja2
* urllib3
* requests
* cryptography

environment.yaml

@@ -24,6 +24,6 @@ channels:
 
 dependencies:
   - pip
-  - python=3.6
+  - python=3
   - sphinx
   - sphinx_rtd_theme

requirements.txt

@@ -1,29 +1,35 @@
 # Python packages and versions used in recent docs development environment
+#
+# Create/update this file with:
+#   (moad-docs)$ python3 -m pip freeze --path $CONDA_PREFIX/lib/python3*/site-packages >> requirements.txt
 
-alabaster==0.7.10
-asn1crypto==0.24.0
-Babel==2.5.3
-certifi==2018.1.18
-cffi==1.11.5
+alabaster==0.7.12
+Babel==2.8.0
+certifi==2019.11.28
+cffi==1.13.2
 chardet==3.0.4
-cryptography==2.1.4
-docutils==0.14
-idna==2.6
-imagesize==1.0.0
-Jinja2==2.10
-MarkupSafe==1.0
-packaging==17.1
-pycparser==2.18
-Pygments==2.2.0
-pyOpenSSL==17.5.0
-pyparsing==2.2.0
-PySocks==1.6.8
-pytz==2018.3
-requests==2.18.4
-six==1.11.0
-snowballstemmer==1.2.1
-Sphinx==1.7.1
-sphinx-rtd-theme==0.2.4
-sphinxcontrib-websupport==1.0.1
-typing==3.6.4
-urllib3==1.22
+cryptography==2.8
+docutils==0.16
+idna==2.9
+imagesize==1.2.0
+Jinja2==2.11.1
+MarkupSafe==1.1.1
+packaging==20.1
+pycparser==2.19
+Pygments==2.5.2
+pyOpenSSL==19.1.0
+pyparsing==2.4.6
+PySocks==1.7.1
+pytz==2019.3
+requests==2.23.0
+six==1.14.0
+snowballstemmer==2.0.0
+Sphinx==2.4.3
+sphinx-rtd-theme==0.4.3
+sphinxcontrib-applehelp==1.0.1
+sphinxcontrib-devhelp==1.0.1
+sphinxcontrib-htmlhelp==1.0.3
+sphinxcontrib-jsmath==1.0.1
+sphinxcontrib-qthelp==1.0.2
+sphinxcontrib-serializinghtml==1.1.3
+urllib3==1.25.7