Add pdfminer.six to minimum version testing

20181108 is the version used by the current Debian oldstable.
Ubuntu 20.04 LTS has 20191020.

pdfminer installs cryptography, >= 38 breaks pyOpenSSL < 22 as found in
Ubuntu 20.04 LTS. Update docs job to use ubuntu-22.04 to avoid
failures.

.github/workflows/build.yml

@@ -84,7 +84,7 @@ jobs:
     # there's some overlap with publish-pages.yml, but this one runs on pull
     # requests, doesn't publish, and runs more doc checks
     name: docs
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
 
     steps:
       - name: Git clone

requirements-min.txt

@@ -4,3 +4,4 @@ requests == 2.20.0
 dnspython == 2.0.0
 # optional:
 argcomplete == 1.8.1
+pdfminer.six == 20181108

requirements.txt

@@ -4,3 +4,4 @@ requests >= 2.20
 dnspython >= 2.0
 # optional:
 argcomplete >= 1.8.1
+pdfminer.six >= 20181108

tox.ini

@@ -5,7 +5,6 @@ envlist = py3{7,8,9,10,11}, minreqs
 deps =
     pyftpdlib
     parameterized
-    pdfminer.six
     pyopenssl
     pytest-xdist
     pytest-cov