chore(tox): bump version to 3.21.2

* drop python 2.7 support
* add python 3.9 support

tox/Dockerfile

@@ -1,18 +1,18 @@
 FROM alpine:3
 
 ENV PATH="$PATH:/root/.pyenv/bin:/root/.pyenv/shims"
-ENV version=3.20.0
+ENV version=3.21.2
 
 RUN apk add --no-cache --virtual=.build-deps curl git linux-headers openssl-dev sqlite-dev readline-dev bzip2-dev ncurses-dev sqlite-dev patch xz-dev zlib-dev && \
     apk add --no-cache --virtual=.run-deps bash build-base curl-dev openssl readline libffi libbz2 libffi-dev bzip2 ncurses sqlite sqlite-libs zlib xz postgresql-dev ca-certificates && \
     curl --location https://raw.githubusercontent.com/pyenv/pyenv-installer/master/bin/pyenv-installer | bash && \
     pyenv update && \
-    pyenv install 2.7.18 && \
     pyenv install 3.5.10 && \
     pyenv install 3.6.12 && \
     pyenv install 3.7.9 && \
-    pyenv install 3.8.5 && \
-    pyenv global 3.8.5 3.7.9 3.6.12 3.5.10 2.7.18 && \
+    pyenv install 3.8.7 && \
+    pyenv install 3.9.1 && \
+    pyenv global 3.9.1 3.8.7 3.7.9 3.6.12 3.5.10 && \
     pyenv rehash && \
     pip install tox==$version && \
     apk del .build-deps && \