dev env: add typed-ast to fix mypy on python3.7

```
$ pipenv run mypy "$(cat *.egg-info/top_level.txt)" tests
The typed_ast package is not installed.
You can install it with `python3 -m pip install typed-ast`.
Error: Process completed with exit code 1.
```

https://github.com/fphammerle/ical2vdir/commit/5e394675dc411051fa5a19d971abe26cf6de16d8

Pipfile

@@ -14,6 +14,9 @@ pylint-import-requirements = "*"
 pytest = "*"
 pytest-cov = "*"
 
+# mypy on python<3.8
+typed-ast = {markers = "python_version < '3.8'"}
+
 [requires]
 python_version = "3"
 

Pipfile.lock

@@ -1,7 +1,7 @@
 {
     "_meta": {
         "hash": {
-            "sha256": "90bfc570c26222eaf802a7716b38a0c200b4ba967d0bf9a77b10f711ad059f44"
+            "sha256": "d97faab2587c164e20f751a548bf6c3603209b9cd4f4de8c1293aba9721c2910"
         },
         "pipfile-spec": 6,
         "requires": {
@@ -357,6 +357,37 @@
             "markers": "python_version >= '3.7'",
             "version": "==2.0.1"
         },
+        "typed-ast": {
+            "hashes": [
+                "sha256:0eb77764ea470f14fcbb89d51bc6bbf5e7623446ac4ed06cbd9ca9495b62e36e",
+                "sha256:1098df9a0592dd4c8c0ccfc2e98931278a6c6c53cb3a3e2cf7e9ee3b06153344",
+                "sha256:183b183b7771a508395d2cbffd6db67d6ad52958a5fdc99f450d954003900266",
+                "sha256:18fe320f354d6f9ad3147859b6e16649a0781425268c4dde596093177660e71a",
+                "sha256:26a432dc219c6b6f38be20a958cbe1abffcc5492821d7e27f08606ef99e0dffd",
+                "sha256:294a6903a4d087db805a7656989f613371915fc45c8cc0ddc5c5a0a8ad9bea4d",
+                "sha256:31d8c6b2df19a777bc8826770b872a45a1f30cfefcfd729491baa5237faae837",
+                "sha256:33b4a19ddc9fc551ebabca9765d54d04600c4a50eda13893dadf67ed81d9a098",
+                "sha256:42c47c3b43fe3a39ddf8de1d40dbbfca60ac8530a36c9b198ea5b9efac75c09e",
+                "sha256:525a2d4088e70a9f75b08b3f87a51acc9cde640e19cc523c7e41aa355564ae27",
+                "sha256:58ae097a325e9bb7a684572d20eb3e1809802c5c9ec7108e85da1eb6c1a3331b",
+                "sha256:676d051b1da67a852c0447621fdd11c4e104827417bf216092ec3e286f7da596",
+                "sha256:74cac86cc586db8dfda0ce65d8bcd2bf17b58668dfcc3652762f3ef0e6677e76",
+                "sha256:8c08d6625bb258179b6e512f55ad20f9dfef019bbfbe3095247401e053a3ea30",
+                "sha256:90904d889ab8e81a956f2c0935a523cc4e077c7847a836abee832f868d5c26a4",
+                "sha256:963a0ccc9a4188524e6e6d39b12c9ca24cc2d45a71cfdd04a26d883c922b4b78",
+                "sha256:bbebc31bf11762b63bf61aaae232becb41c5bf6b3461b80a4df7e791fabb3aca",
+                "sha256:bc2542e83ac8399752bc16e0b35e038bdb659ba237f4222616b4e83fb9654985",
+                "sha256:c29dd9a3a9d259c9fa19d19738d021632d673f6ed9b35a739f48e5f807f264fb",
+                "sha256:c7407cfcad702f0b6c0e0f3e7ab876cd1d2c13b14ce770e412c0c4b9728a0f88",
+                "sha256:da0a98d458010bf4fe535f2d1e367a2e2060e105978873c04c04212fb20543f7",
+                "sha256:df05aa5b241e2e8045f5f4367a9f6187b09c4cdf8578bb219861c4e27c443db5",
+                "sha256:f290617f74a610849bd8f5514e34ae3d09eafd521dceaa6cf68b3f4414266d4e",
+                "sha256:f30ddd110634c2d7534b2d4e0e22967e88366b0d356b24de87419cc4410c41b7"
+            ],
+            "index": "pypi",
+            "markers": "python_version < '3.8'",
+            "version": "==1.5.2"
+        },
         "typing-extensions": {
             "hashes": [
                 "sha256:4ca091dea149f945ec56afb48dae714f21e8692ef22a395223bcd328961b6a0e",