Fail reading package-infos with unexpected encodings

Since the `PKG-INFO` file etc frequently contain the full README.rst,
it happens that strange characters are contained.

Unfortunately, packaging-tools do not check encoding-validity of chars.
So it is unpleasent when later `pkginfo` cannot extract metadata.

Roughlt the following (hand-made) patch would suffice:

```diff

--- a/pkginfo/develop.py
+++ b/pkginfo/develop.py
@@ -0,2 +0,3 @@
+import io
 import os
 import sys

@@ -41,3 +41,3 @@
             if os.path.exists(path):
- with open(path) as f:
+ with io.open(path, errors='ignore') as f:
                     return f.read()

```

Respective fixes needed in all other modules (e.g. `pkginfo.installed.py`).