env: downgrade astroid to restore compatibility with python3.4

https://travis-ci.org/fphammerle/free-disk/jobs/630908771#L209

Pipfile

@@ -7,6 +7,9 @@ name = "pypi"
 free-disk = {editable = true, path = "."}
 
 [dev-packages]
+# v2.3.0 dropped python3.4 support
+# https://github.com/PyCQA/astroid/commit/4a4b7faf9de8437f2b96bc7285802fd1230b7a9a
+astroid = "<2.3.0"
 # v8.0.0 dropped python3.4 support
 more-itertools = "<8"
 # v2.4.0 dropped python3.4 support

Pipfile.lock

@@ -1,7 +1,7 @@
 {
     "_meta": {
         "hash": {
-            "sha256": "70a80c9b86b4ccddae08d2c528a756c9411cda321dc664fc335d245c6422b5cd"
+            "sha256": "512e7f5f7622fafcb1ece27ac220fd6643e9bebcdb2c4d6afb8fd62a534aa9e7"
         },
         "pipfile-spec": 6,
         "requires": {
@@ -24,10 +24,11 @@
     "develop": {
         "astroid": {
             "hashes": [
-                "sha256:71ea07f44df9568a75d0f354c49143a4575d90645e9fead6dfb52c26a85ed13a",
-                "sha256:840947ebfa8b58f318d42301cf8c0a20fd794a33b61cc4638e28e9e61ba32f42"
+                "sha256:6560e1e1749f68c64a4b5dee4e091fce798d2f0d84ebe638cf0e0585a343acf4",
+                "sha256:b65db1bbaac9f9f4d190199bb8680af6f6f84fd3769a5ea883df8a91fe68b4c4"
             ],
-            "version": "==2.3.3"
+            "index": "pypi",
+            "version": "==2.2.5"
         },
         "atomicwrites": {
             "hashes": [
@@ -173,7 +174,7 @@
                 "sha256:fdc1c9bbf79510b76408840e009ed65958feba92a88833cdceecff93ae8fff66",
                 "sha256:ffde2fbfad571af120fcbfbbc61c72469e72f550d676c3342492a9dfdefb8f12"
             ],
-            "markers": "implementation_name == 'cpython' and python_version < '3.8'",
+            "markers": "implementation_name == 'cpython'",
             "version": "==1.4.0"
         },
         "wcwidth": {