Split out version markers in requirements.txt (#53)

This is a valid line in a requirements.txt:

```
appnope==0.1.4 ; sys_platform == 'darwin' or platform_system == 'Darwin'
```

Previously this would fail because of multiple instances of `==`.
Dropping everything after the `;` should be safe, and doesn't cause
ambiguity for lines like:

```
requirement~=0.1.4 ; sys_platform == 'darwin'
```

Which could split on `~=` or `==`

mypy/private/types.bzl

diff --git a/mypy/private/types.bzl b/mypy/private/types.bzl
index e1a03ab..5e4d1ca 100644
--- a/mypy/private/types.bzl
+++ b/mypy/private/types.bzl
@@ -46,6 +46,9 @@
         if line.startswith("#") or line == "":
             continue
 
+        if ";" in line:
+            line, _ = line.split(";")
+
         if "~=" in line:
             req, _ = line.split("~=")
         elif "==" in line: