NSIS: Remove installation dir if it exists

If we don't do this, when a newer version is installed on top of an older one,
old files which have been deleted persist. In the case of v1.3.3 -> v1.4.0,
this means qutebrowser won't start because of a leftover sip installation which
confuses PyQt 5.11.

This is still potentially dangerous as the user could use e.g. C:\ as
installation path, but we have that issue when uninstalling anyways.
See http://nsis.sourceforge.net/Uninstall_only_installed_files

.github/CONTRIBUTING.asciidoc

@@ -1,3 +1,8 @@
+IMPORTANT: I'm currently (July 2018) more busy than usual until September,
+because of exams coming up. Review of non-trivial pull requests will thus be
+delayed until then. If you're reading this note after mid-September, please
+open an issue.
+
 - Before you start to work on something, please leave a comment on the relevant
   issue (or open one). This makes sure there is no duplicate work done.
 

.travis.yml

@@ -65,10 +65,6 @@ matrix:
       env: TESTENV=shellcheck
       services: docker
   fast_finish: true
-  allow_failures:
-    # https://github.com/qutebrowser/qutebrowser/issues/4055
-    - os: linux
-      env: TESTENV=py36-pyqt510
 
 cache:
   directories:

doc/contributing.asciidoc

@@ -5,6 +5,11 @@ The Compiler <mail@qutebrowser.org>
 :data-uri:
 :toc:
 
+IMPORTANT: I'm currently (July 2018) more busy than usual until September,
+because of exams coming up. Review of non-trivial pull requests will thus be
+delayed until then. If you're reading this note after mid-September, please
+open an issue.
+
 I `&lt;3` footnote:[Of course, that says `<3` in HTML.] contributors!
 
 This document contains guidelines for contributing to qutebrowser, as well as

misc/qutebrowser.nsi

@@ -40,6 +40,8 @@ Section "Install"
   ; Uninstall old versions
   ExecWait 'MsiExec.exe /quiet /qn /norestart /X{633F41F9-FE9B-42D1-9CC4-718CBD01EE11}'
   ExecWait 'MsiExec.exe /quiet /qn /norestart /X{9331D947-AC86-4542-A755-A833429C6E69}'
+  RMDir /r "$INSTDIR\*.*"
+  CreateDirectory "$INSTDIR"
 
   SetOutPath "$INSTDIR"
   

scripts/link_pyqt.py

@@ -136,15 +136,7 @@ def link_pyqt(executable, venv_path):
         executable: The python executable where the source files are present.
         venv_path: The path to the virtualenv site-packages.
     """
-    try:
-        get_lib_path(executable, 'PyQt5.sip')
-    except Error:
-        # There is no PyQt5.sip, so we need to copy the toplevel sip.
-        sip_file = get_lib_path(executable, 'sip')
-    else:
-        # There is a PyQt5.sip, it'll get copied with the PyQt5 dir.
-        sip_file = None
-
+    sip_file = get_lib_path(executable, 'sip')
     sipconfig_file = get_lib_path(executable, 'sipconfig', required=False)
     pyqt_dir = os.path.dirname(get_lib_path(executable, 'PyQt5.QtCore'))