Add --skip-incompatible flag

This flag makes it possible to skip requirements that are incompatible
with the current environment, i.e. those whose PEP-496 environment
markers do not evaluate thruthly.

For example, when running Python 3.5, previously there would be no way
of processing the following requirement properly:

```
enum34; python_version<"3.4"
```

Ignoring it via `-r` is a bit heavy handed, as it means that it would no
longer get checked when running e.g. Python 2.7.

By using `-s` or `--skip-incompatible`, this requirement gets ignored
when running python versions 3.4 and above, but evaluated when running
on earlier versions.

Author: MrMino

pip_check_reqs/common.py

@@ -6,6 +6,7 @@
 import re
 
 from packaging.utils import canonicalize_name
+from packaging.markers import Marker
 # Between different versions of pip the location of PipSession has changed.
 try:
     from pip._internal.network.session import PipSession
@@ -142,12 +143,33 @@ def find_required_modules(options, requirements_filename: str):
 
         if options.ignore_reqs(requirement):
             log.debug('ignoring requirement: %s', requirement_name)
+            continue
+
+        if options.skip_incompatible:
+            requirement_string = requirement.requirement
+            if not has_compatible_markers(requirement_string):
+                log.debug('ignoring requirement (incompatible environment '
+                          'marker): %s', requirement_string)
+                continue
+
         else:
             log.debug('found requirement: %s', requirement_name)
             explicit.add(canonicalize_name(requirement_name))
+
     return explicit
 
 
+def has_compatible_markers(full_requirement: str) -> bool:
+    if ';' not in full_requirement:
+        return True  # No environment marker.
+
+    enviroment_marker = full_requirement.split(';')[1]
+    if not enviroment_marker:
+        return True  # Empty environment marker.
+
+    return Marker(enviroment_marker).evaluate()
+
+
 def is_package_file(path):
     '''Determines whether the path points to a Python package sentinel
     file - the __init__.py or its compiled variants.

pip_check_reqs/find_extra_reqs.py

@@ -87,6 +87,13 @@ def main():
                       action="append",
                       default=[],
                       help="reqs in requirements to ignore")
+    parser.add_option("-s",
+                      "--skip-incompatible",
+                      dest="skip_incompatible",
+                      action="store_true",
+                      default=False,
+                      help="skip requirements that have incompatible "
+                           "environment markers")
     parser.add_option("-v",
                       "--verbose",
                       dest="verbose",